USMLE Step 1 immunology questions

Chediak-Higashi syndrome results from a mutation in the LYST gene, which encodes a protein required for normal lysosomal trafficking and microtubule-dependent fusion of lysosomes with phagosomes. This defect impairs the ability of neutrophils to deliver microbicidal enzymes to engulfed organisms, resulting in giant, poorly functional cytoplasmic granules and recurrent pyogenic infections. The syndrome also features partial albinism (due to abnormal melanosome trafficking) and progressive peripheral neuropathy. Giant granules on peripheral smear in a child with albinism and recurrent infections are pathognomonic.

• B. This describes leukocyte adhesion deficiency type 1, which does not cause giant granules or albinism.
• C. This describes chronic granulomatous disease, which does not feature giant granules or albinism.
• D. This describes terminal complement deficiency, which predisposes to Neisseria infections, not this phenotype.
• E. This describes hereditary angioedema from C1 esterase inhibitor deficiency, unrelated to this presentation.

This infant has leukocyte adhesion deficiency type 1 (LAD-1), caused by a defect in the gene encoding CD18, the common beta-2 subunit shared by integrins (LFA-1, Mac-1) required for firm adhesion of neutrophils to vascular endothelium. Without functional integrins, neutrophils cannot leave the bloodstream to reach sites of infection, causing marked peripheral neutrophilia despite absent neutrophil infiltration at infection sites. Delayed separation of the umbilical cord beyond 3-4 weeks and poor wound healing are classic clinical clues. Recurrent skin and mucosal infections without pus formation are characteristic because neutrophils cannot extravasate.

• A. CD55/CD59 deficiency causes paroxysmal nocturnal hemoglobinuria, presenting with hemolysis and thrombosis, not delayed cord separation.
• B. IL-12 deficiency impairs Th1 responses and predisposes to mycobacterial infections, not delayed cord separation.
• C. Myeloperoxidase deficiency does not impair neutrophil migration or cause delayed cord separation.
• D. Leukocyte adhesion deficiency type 2 involves defective selectin ligands and presents similarly but is far rarer and not the classic answer for this vignette.

This patient has hereditary angioedema due to C1 esterase inhibitor deficiency, which normally restrains the complement, kinin, and coagulation cascades, and its deficiency leads to unchecked production of bradykinin, causing recurrent, non-pruritic angioedema and self-limited abdominal pain from bowel wall edema. ACE inhibitors like lisinopril block the breakdown of bradykinin by angiotensin-converting enzyme and are absolutely contraindicated in these patients because they can precipitate life-threatening laryngeal edema. Low C4 levels during attacks reflect ongoing complement activation and consumption, and are used along with low C1 inhibitor levels to confirm the diagnosis. Angiotensin receptor blockers like losartan do not affect bradykinin metabolism and are safer alternatives.

• A. Amlodipine, a calcium channel blocker, does not affect bradykinin metabolism and is not contraindicated.
• C. Hydrochlorothiazide, a diuretic, does not affect bradykinin levels and is not contraindicated.
• D. Losartan, an angiotensin receptor blocker, does not inhibit ACE and does not increase bradykinin levels.
• E. Metoprolol, a beta-blocker, does not affect bradykinin metabolism and is not contraindicated.

X-linked agammaglobulinemia is caused by a mutation in Bruton tyrosine kinase (BTK), a signaling enzyme required for pre-B cell receptor signaling that allows progression from the pre-B cell stage to mature B cells. Without BTK, B-cell development arrests in the bone marrow, resulting in absent or markedly decreased circulating B cells (CD19+), absent lymphoid tissue (tonsils, lymph nodes), and panhypogammaglobulinemia. Boys present after maternal antibody wanes (around 6 months) with recurrent infections from encapsulated bacteria and enteroviruses. T-cell numbers and function are normal because T-cell development is independent of BTK.

• B. This defect impairs both B- and T-cell receptor diversification and causes conditions like Omenn syndrome, not isolated B-cell arrest.
• C. This defect causes X-linked SCID, which affects T and NK cell development with normal or increased B cells.
• D. This defect causes adenosine deaminase deficiency, a cause of SCID with both low B and T cells.
• E. This defect causes hyper-IgM syndrome, with normal B-cell numbers but elevated IgM and low IgG/IgA.

This patient has common variable immunodeficiency (CVID), characterized by low immunoglobulin levels (IgG, often IgA and IgM) with normal B-cell numbers but impaired differentiation into plasma cells, usually presenting in the 20s-40s with recurrent sinopulmonary infections and increased susceptibility to Giardia. CVID carries a significantly increased risk of lymphoid malignancies (particularly B-cell non-Hodgkin lymphoma) and autoimmune disease (such as autoimmune cytopenias), thought to result from dysregulated lymphocyte proliferation and chronic immune activation. Patients require lifelong monitoring for these complications in addition to immunoglobulin replacement.

• A. Complement deficiencies cause recurrent Neisseria infections, not a feature of CVID.
• B. Invasive aspergillosis is a hallmark of chronic granulomatous disease and neutrophil disorders, not CVID.
• D. Hereditary angioedema results from C1 esterase inhibitor deficiency, unrelated to CVID.
• E. Chronic granulomatous disease is a distinct NADPH oxidase defect, not part of CVID.

Hyper-IgM syndrome is most commonly X-linked and caused by a mutation in CD40 ligand (CD40L/CD154) on activated T cells. Normally, CD40L on T cells binds CD40 on B cells to trigger immunoglobulin class switching from IgM to IgG, IgA, and IgE; without this signal, B cells remain restricted to producing IgM, resulting in normal or elevated IgM with low levels of other isotypes. The defect also impairs macrophage activation (since CD40-CD40L signaling helps activate macrophages), explaining susceptibility to Pneumocystis jirovecii and other opportunistic infections, as well as pyogenic infections from poor opsonization.

• B. This defect causes bare lymphocyte syndrome type II with absent CD4+ T cells.
• C. This defect causes chronic granulomatous disease with recurrent catalase-positive organism infections.
• D. This defect causes X-linked SCID with absent T and NK cells.
• E. This defect causes X-linked agammaglobulinemia with absent B cells, not hyper-IgM syndrome.

This infant has DiGeorge syndrome, caused by a 22q11.2 microdeletion resulting in failure of the third and fourth pharyngeal pouches to develop properly. This produces thymic hypoplasia or aplasia (T-cell deficiency), parathyroid hypoplasia (hypocalcemia from hypoparathyroidism), and conotruncal cardiac defects, along with characteristic facial features. Because B-cell development is unaffected, immunoglobulin levels and B-cell numbers are typically normal, while absolute T-cell counts are decreased due to impaired thymic education and export of T cells.

• A. This describes ataxia-telangiectasia, which presents later in childhood with cerebellar ataxia, not neonatal hypocalcemia.
• B. This pattern describes SCID, which lacks the characteristic facial and cardiac findings of 22q11 deletion.
• C. This describes X-linked agammaglobulinemia, which does not feature hypocalcemia or cardiac defects.
• E. This describes hyper-IgM syndrome, unrelated to thymic aplasia or hypoparathyroidism.

Infant B has autosomal recessive SCID due to adenosine deaminase (ADA) deficiency, which causes accumulation of deoxyadenosine and its toxic metabolite dATP within lymphocytes, impairing DNA synthesis and killing developing T, B, and NK cells (since ADA deficiency affects all lymphoid lineages). In contrast, infant A has the more common X-linked SCID caused by a mutation in the common gamma chain, a shared subunit of several interleukin receptors (including IL-2, IL-4, IL-7, IL-9, IL-15, IL-21) that is required for T-cell and NK-cell development, leaving B cells numerically normal but nonfunctional due to lack of T-cell help.

• A. This mechanism describes infant A's X-linked SCID, which spares B-cell numbers.
• C. MHC class I deficiency causes bare lymphocyte syndrome type I with isolated CD8+ T-cell deficiency, not pan-lymphocyte depletion.
• D. RAG mutations impair both T- and B-cell receptor recombination, not B cells alone, and would not selectively explain infant B's pattern.
• E. This defect causes X-linked agammaglobulinemia with normal T cells, not SCID.

This child has ataxia-telangiectasia, caused by mutation of the ATM gene, which encodes a kinase essential for recognizing and repairing double-stranded DNA breaks. Loss of ATM function leads to cerebellar degeneration (progressive ataxia beginning in early childhood), oculocutaneous telangiectasias, immunodeficiency (often with IgA deficiency and recurrent sinopulmonary infections), and an increased risk of lymphoid malignancy due to genomic instability. Elevated serum alpha-fetoprotein is a characteristic and clinically useful diagnostic marker in ataxia-telangiectasia, although the mechanism for its elevation is not fully understood.

• B. Elevated creatine kinase suggests a muscular dystrophy, which does not explain the telangiectasias or immunodeficiency.
• C. A positive ANA suggests a systemic autoimmune disease such as lupus, not ataxia-telangiectasia.
• D. Decreased ceruloplasmin suggests Wilson disease, which causes movement disorders but not telangiectasias or immunodeficiency.
• E. Elevated lactate and pyruvate suggest a mitochondrial disorder, not ataxia-telangiectasia.

This boy has Wiskott-Aldrich syndrome, an X-linked recessive disorder caused by mutation of the WAS gene, which encodes WASP, a protein that regulates actin cytoskeleton reorganization in hematopoietic cells, including platelets and lymphocytes. The classic triad is thrombocytopenia with small platelets, eczema, and recurrent infections (due to a progressive immunodeficiency with poor response to polysaccharide antigens), and the mnemonic WIPE (Wiskott-Aldrich, Infections, Purpura/thrombocytopenia, Eczema) is often used. Immunoglobulin abnormalities typically show low IgM with normal or elevated IgA and IgE.

• A. This defect causes X-linked SCID, not the triad described here.
• C. This describes DiGeorge syndrome, which does not cause thrombocytopenia or eczema.
• D. This defect causes ataxia-telangiectasia, which presents with ataxia and telangiectasias rather than thrombocytopenia and eczema in infancy.
• E. This defect causes X-linked agammaglobulinemia, which does not cause thrombocytopenia or eczema.

Full T-cell activation requires two signals: signal 1 is TCR recognition of peptide-MHC, and signal 2 is costimulation via CD28 on the T cell binding B7 (CD80/86) on the antigen-presenting cell. When signal 1 occurs without signal 2, the T cell becomes anergic (functionally unresponsive) rather than activated, a mechanism that helps maintain peripheral tolerance and prevent autoimmunity. Anergic T cells survive but fail to proliferate or produce IL-2 upon subsequent antigen encounter. This two-signal requirement is exploited therapeutically by drugs like abatacept, which blocks CD28-B7 interaction.

• B. TCR-MHC engagement alone (signal 1) without costimulation induces anergy, not normal activation.
• C. Regulatory T-cell differentiation requires specific cytokine signals (e.g., TGF-beta), not simply absent costimulation.
• D. Loss of costimulation does not enhance proliferation; it prevents full activation.
• E. Negative selection is a central tolerance mechanism occurring in the thymus, not a peripheral outcome of blocked costimulation.

Type I hypersensitivity reactions begin when antigen (peanut protein) cross-links preformed IgE antibodies bound to high-affinity Fc-epsilon-RI receptors on the surface of sensitized mast cells and basophils. This cross-linking triggers immediate degranulation, releasing preformed mediators such as histamine and tryptase, followed by synthesis of leukotrienes and prostaglandins, producing vasodilation, increased vascular permeability, bronchoconstriction, and hypotension. This occurs within minutes of re-exposure in a previously sensitized individual. Epinephrine reverses the reaction by causing vasoconstriction, bronchodilation, and increasing cardiac output via alpha-1, beta-1, and beta-2 adrenergic receptor activation.

• B. Direct complement activation without antibody involvement is not the mechanism of IgE-mediated anaphylaxis.
• C. Immune complex deposition describes type III hypersensitivity, which has a delayed onset of hours and does not cause immediate anaphylaxis.
• D. Cytotoxic T-cell formation is a type IV, cell-mediated mechanism with delayed onset, not an immediate reaction.
• E. IgG binding to cell-surface antigens describes type II hypersensitivity, not mast cell degranulation.

Goodpasture syndrome is a type II hypersensitivity reaction in which autoantibodies, usually IgG, are directed against the non-collagenous (NC1) domain of the alpha-3 chain of type IV collagen, found in the glomerular and alveolar basement membranes. Antibody binding activates complement and recruits inflammatory cells, producing linear immunofluorescent staining and crescentic (rapidly progressive) glomerulonephritis along with pulmonary hemorrhage. This combination of renal and pulmonary findings with linear IgG deposits distinguishes it from immune complex-mediated glomerulonephritides, which show granular deposits.

• A. While T cells contribute to inflammation, the defining mechanism is direct antibody binding to basement membrane antigen, a type II reaction.
• B. ANCA against myeloperoxidase is characteristic of microscopic polyangiitis, which typically shows pauci-immune (no significant immune deposits) glomerulonephritis.
• C. Post-streptococcal glomerulonephritis involves immune complex deposition with granular staining, not linear anti-GBM antibody binding.
• D. Subepithelial immune complex deposition with granular immunofluorescence describes type III-mediated glomerulonephritis such as membranous nephropathy, not Goodpasture syndrome.

Serum sickness is a prototypical type III hypersensitivity reaction that occurs 1-2 weeks after exposure to a foreign protein, such as the horse-derived proteins in antivenom. During this interval, the host generates antibodies (chiefly IgG) that form circulating immune complexes with the residual foreign antigen. These complexes deposit in small vessels, joints, and the renal glomerular basement membrane, activating complement and causing a drop in C3 and C4, along with recruitment of neutrophils that cause tissue damage and manifest as fever, rash, arthralgia, and nephritis. This delayed onset and complement consumption distinguish it from the immediate IgE-mediated reactions of type I hypersensitivity.

• A. Delayed-type T-cell mediated reactions, such as contact dermatitis, do not cause hypocomplementemia or circulating immune complex deposition.
• B. IgE-mediated mast cell degranulation produces immediate reactions (minutes to hours), not a syndrome with a delayed 10-day onset and complement consumption.
• C. Antibody-dependent cellular cytotoxicity is a mechanism in some type II reactions, not in immune complex-mediated serum sickness.
• E. Direct cytotoxic antibody binding to fixed antigens describes type II hypersensitivity, which does not typically consume complement diffusely or cause this multisystem pattern.

The Arthus reaction is a localized type III hypersensitivity reaction that occurs when a person with high levels of preformed IgG antibody is re-exposed to an antigen, such as a vaccine booster, at a local site. Antigen-antibody complexes form locally within small vessels, activate complement, and attract neutrophils, producing fibrinoid necrosis of the vessel wall, local vasculitis, edema, and hemorrhage within hours of exposure. This is distinct from the systemic, circulating immune complex deposition of serum sickness, since the Arthus reaction is confined to the site of antigen entry. The onset within hours and localized nature are characteristic clues.

• A. A pure CD8+ T-cell infiltrate without immune complex or neutrophilic vasculitis would suggest a cell-mediated process, not antibody-mediated vasculitis.
• B. Dense eosinophil and mast cell infiltration without vascular necrosis would be more typical of a type I hypersensitivity reaction.
• D. Linear antibody deposition along the basement membrane is characteristic of type II reactions like Goodpasture syndrome, not local immune complex vasculitis.
• E. Granuloma formation with epithelioid histiocytes reflects chronic type IV hypersensitivity (e.g., tuberculosis), not an acute 8-hour reaction.

The tuberculin skin test is a classic example of a type IV (delayed-type) hypersensitivity reaction. In a previously sensitized individual, intradermal PPD is taken up by local antigen-presenting cells and presented to memory CD4+ T helper cells, which release cytokines, especially interferon-gamma, that recruit and activate macrophages. This produces a palpable, indurated, erythematous reaction that develops over 48 to 72 hours, the standard reading interval for this test. Because BCG vaccination can also sensitize T cells to mycobacterial antigens, induration in a BCG-vaccinated person does not reliably distinguish prior vaccination from true latent infection, and interferon-gamma release assays are often used as a more specific alternative.

• A. An immediate wheal-and-flare reaction within minutes reflects type I hypersensitivity, not the delayed response seen with PPD.
• B. Direct antibody-mediated vascular injury is a type II mechanism and does not describe the PPD response.
• C. Immune complex-mediated painful ulceration within hours would reflect a type III (Arthus-like) reaction, not the expected 48-72 hour delayed-type response.
• E. Granuloma formation with caseous necrosis takes days to weeks to develop histologically and is not visible within hours; it is also not the basis for bedside induration reading.

ANA testing is highly sensitive for SLE (present in over 95% of patients) but has low specificity, as it can be positive in other autoimmune diseases and even in healthy individuals at low titers. Anti-double-stranded DNA (anti-dsDNA) and anti-Smith antibodies are highly specific for SLE, meaning a positive result strongly confirms the diagnosis once other causes have been excluded. Anti-dsDNA titers also correlate with disease activity and risk of lupus nephritis, making them useful for both diagnosis and monitoring. ANA is therefore used as a screening test, while anti-dsDNA or anti-Smith serves as the confirmatory test.

• B. Anti-U1-RNP antibodies are associated with mixed connective tissue disease, not specifically SLE.
• C. Anti-histone antibodies are characteristic of drug-induced lupus, not idiopathic SLE.
• D. Anti-centromere antibodies are associated with limited systemic sclerosis (CREST syndrome).
• E. Rheumatoid factor is associated with rheumatoid arthritis and lacks specificity for SLE.

Hydralazine and procainamide are classic causes of drug-induced lupus, particularly in slow acetylators who metabolize these drugs less efficiently. The proposed mechanism involves drug metabolites inhibiting DNA methylation in T cells, altering gene expression and promoting autoreactivity, as well as forming complexes with nuclear antigens that trigger an immune response. This produces a lupus-like syndrome with arthralgia, myalgia, serositis, and a photosensitive rash, but typically spares the kidneys and central nervous system. Anti-histone antibodies are characteristic, while anti-dsDNA and anti-Smith antibodies, typical of idiopathic SLE, are usually absent; symptoms resolve after discontinuing the offending drug.

• B. Loop diuretics are not a recognized cause of drug-induced lupus.
• C. Beta-blockers are only rarely implicated and are not the classic cause of this presentation.
• D. ACE inhibitors are not a recognized cause of drug-induced lupus.
• E. Calcium channel blockers are not a recognized cause of drug-induced lupus.

Lupus anticoagulant antibodies target phospholipid-protein complexes (such as beta-2 glycoprotein I) that are required as a reagent surface in the PTT assay. In vitro, this binding interferes with the phospholipid component of the test, artificially prolonging the clotting time without correction on mixing study, since the antibody itself inhibits the reaction. In vivo, however, these antibodies promote a hypercoagulable state by activating endothelial cells and platelets, leading to arterial and venous thrombosis and recurrent pregnancy loss. This paradox, a prolonged PTT in a patient with a clinical tendency toward clotting, is the hallmark of antiphospholipid syndrome and lupus anticoagulant.

• A. Lupus anticoagulant antibodies do not enhance antithrombin III activity.
• B. Lupus anticoagulant antibodies do not act through protein C activation.
• C. Clotting factor deficiencies would correct with mixing studies and would cause bleeding rather than thrombosis.
• D. Glycoprotein IIb/IIIa antibodies would impair platelet aggregation and cause bleeding, not the described in vitro/in vivo discordance.

Sjogren syndrome is an autoimmune disorder characterized by lymphocytic infiltration and destruction of exocrine glands, particularly the lacrimal and salivary glands, leading to keratoconjunctivitis sicca (dry eyes) and xerostomia (dry mouth). Labial salivary gland biopsy classically shows focal lymphocytic sialadenitis, with aggregates of lymphocytes surrounding and destroying the gland ducts and acini. This is associated with circulating anti-Ro (SS-A) and anti-La (SS-B) antibodies, which support the diagnosis along with characteristic biopsy findings and reduced tear/saliva production. Chronic glandular destruction also increases the risk of B-cell (marginal zone) lymphoma arising in affected salivary glands.

• B. Amyloid deposition would suggest amyloidosis, not an autoimmune exocrinopathy.
• C. Granulomatous inflammation is not characteristic of Sjogren syndrome and suggests sarcoidosis instead.
• D. Neutrophilic abscess formation suggests acute bacterial sialadenitis rather than a chronic autoimmune process.
• E. Fibrinoid necrosis of vessels suggests a vasculitic process rather than Sjogren syndrome.

Anti-cyclic citrullinated peptide (anti-CCP) antibody is highly specific (greater than 95%) for rheumatoid arthritis, in contrast to rheumatoid factor, which is sensitive but can be positive in other conditions such as Sjogren syndrome, hepatitis C, and even in healthy older adults. Anti-CCP antibodies target citrullinated proteins generated during inflammation and often appear years before clinical symptoms develop, making them useful for early and more specific diagnosis. The presence of anti-CCP also correlates with more aggressive, erosive joint disease and helps distinguish rheumatoid arthritis from other causes of inflammatory polyarthritis. Combining rheumatoid factor with anti-CCP improves diagnostic specificity over either test alone.

• A. Anti-Jo-1 antibody is associated with antisynthetase syndrome (polymyositis/dermatomyositis), not rheumatoid arthritis.
• B. Antinuclear antibody is sensitive for SLE but not specific for rheumatoid arthritis.
• C. p-ANCA is associated with microscopic polyangiitis and eosinophilic granulomatosis with polyangiitis, not rheumatoid arthritis.
• E. Anti-Scl-70 antibody is associated with diffuse systemic sclerosis, not rheumatoid arthritis.

Myasthenia gravis is a type II hypersensitivity reaction in which autoantibodies bind postsynaptic nicotinic acetylcholine receptors at the neuromuscular junction. This binding causes receptor internalization and degradation, and activates complement to damage the postsynaptic membrane, reducing the number of functional receptors available for acetylcholine binding. The result is progressive weakness with repeated muscle use (fatigability) as available receptors become relatively insufficient, classically affecting ocular and bulbar muscles first. Repetitive nerve stimulation shows a decremental response, and the condition is often associated with thymic hyperplasia or thymoma.

• A. Sodium channel blockade is not the mechanism of myasthenia gravis.
• B. Myasthenia gravis is an antibody-mediated postsynaptic disease, not a demyelinating neuropathy.
• C. Myasthenia gravis does not involve destruction of anterior horn motor neurons.
• D. This describes the mechanism of Lambert-Eaton myasthenic syndrome, not myasthenia gravis.

Graves disease is caused by autoantibodies, specifically thyroid-stimulating immunoglobulins, that bind to and activate the TSH receptor on thyroid follicular cells. This produces a type II hypersensitivity mechanism of receptor stimulation rather than destruction, mimicking the action of TSH and causing continuous, unregulated synthesis and release of thyroid hormone independent of normal feedback control. The resulting excess thyroid hormone suppresses pituitary TSH secretion through negative feedback, producing the characteristic laboratory pattern of low TSH with elevated free T4. Associated findings include diffuse goiter, exophthalmos from retro-orbital inflammation, and a hypermetabolic clinical picture.

• A. Antibodies against thyroid peroxidase are seen in Hashimoto thyroiditis, not as the primary driver of Graves disease.
• B. Graves disease is mediated by stimulating autoantibodies, not immune complex deposition.
• C. This describes thyroiditis (such as the destructive phase of Hashimoto thyroiditis), not Graves disease.
• E. The antibodies in Graves disease stimulate rather than block the TSH receptor.

This patient has ankylosing spondylitis, a seronegative spondyloarthropathy strongly associated with HLA-B27, a class I MHC molecule. One proposed mechanism is molecular mimicry, in which HLA-B27 presents self-peptides that structurally resemble bacterial antigens (e.g., from Klebsiella or enteric pathogens), triggering a cross-reactive cytotoxic T-cell response against joint and entheseal tissue. Seronegative spondyloarthropathies (ankylosing spondylitis, reactive arthritis, psoriatic arthritis, enteropathic arthritis) are characteristically rheumatoid factor-negative and linked to HLA-B27 rather than class II alleles. Enthesitis, as seen here at the Achilles insertion, is a hallmark feature distinguishing these disorders from rheumatoid arthritis.

• B. AIRE mutations cause autoimmune polyendocrine syndrome type 1, unrelated to this presentation.
• C. Complement deficiency predisposes to SLE-like disease, not seronegative spondyloarthropathy.
• D. Class II MHC and anti-citrullinated peptide antibodies are implicated in rheumatoid arthritis, which this patient does not have (RF negative).
• E. This describes a mechanism relevant to lymphoma or autoimmune cytopenias, not spondyloarthropathy.

This patient's presentation of upper respiratory tract involvement (nasal crusting), cavitary pulmonary nodules, and rapidly progressive glomerulonephritis is classic for granulomatosis with polyangiitis (GPA). GPA is associated with c-ANCA, which produces a cytoplasmic immunofluorescence pattern and targets proteinase 3 (PR3), an enzyme found in neutrophil azurophilic granules. In contrast, microscopic polyangiitis and eosinophilic granulomatosis with polyangiitis are associated with p-ANCA (perinuclear pattern), which targets myeloperoxidase (MPO) and typically lacks granulomatous upper airway and cavitary lung disease. Distinguishing PR3/c-ANCA from MPO/p-ANCA helps differentiate these small-vessel vasculitides clinically and serologically.

• A. Anti-smooth muscle antibodies are associated with autoimmune hepatitis, unrelated to this vasculitic presentation.
• B. Anti-dsDNA is associated with systemic lupus erythematosus, not ANCA-associated vasculitis.
• C. Anti-GBM antibodies cause Goodpasture syndrome, which is not ANCA-mediated and does not produce this cytoplasmic staining pattern.
• E. Myeloperoxidase produces a perinuclear (p-ANCA) pattern and is associated with microscopic polyangiitis, which lacks the granulomatous cavitary and upper airway findings seen here.

Hyperacute rejection occurs within minutes to hours of transplantation due to preformed recipient antibodies (e.g., against ABO blood group antigens or donor HLA) that bind donor endothelial antigens immediately upon reperfusion. This triggers complement activation, endothelial damage, and a procoagulant state, leading to widespread thrombosis and fibrinoid necrosis of vessel walls, causing immediate graft ischemia and infarction. Because the damage is vascular and antibody-mediated rather than cellular, it occurs too rapidly for a T-cell infiltrate to develop. Hyperacute rejection is now rare due to routine pretransplant crossmatching to detect preformed donor-specific antibodies.

• A. A dense CD8+ lymphocytic infiltrate describes acute cellular rejection, which develops over weeks, not minutes.
• B. Concentric intimal thickening describes chronic rejection, which develops over months to years.
• D. Dermal and bile duct lymphocytic infiltration describes graft-versus-host disease, not hyperacute rejection of a solid organ.
• E. Granulomatous inflammation is not a feature of hyperacute rejection.

Acute cellular rejection typically occurs weeks to months after transplantation and is mediated by recipient CD8+ cytotoxic T lymphocytes that become sensitized to donor MHC class I (and CD4+ T cells to class II) molecules presented on graft cells. These cytotoxic T cells directly attack graft parenchyma, producing the characteristic histologic finding of lymphocytic interstitial infiltration with tubulitis (lymphocytes invading tubular epithelium) in a renal allograft. This process is reversible with increased immunosuppression (e.g., high-dose corticosteroids or increased calcineurin inhibitor dosing) if caught early. This contrasts with hyperacute rejection (preformed antibody, minutes) and chronic rejection (vascular fibrosis, months to years).

• A. Donor T cells attacking recipient tissue describes graft-versus-host disease, which occurs after bone marrow or stem cell transplantation, not solid organ transplantation.
• C. Preformed antibody-mediated complement activation describes hyperacute rejection, which occurs within minutes, not weeks.
• D. Granulomatous inflammation is not the mechanism of acute cellular rejection.
• E. Gradual antibody and complement deposition causing vascular fibrosis describes chronic rejection, which develops over months to years.

Chronic rejection develops over months to years and results from persistent low-grade immune injury to the graft vasculature, involving both antibody-mediated and cell-mediated mechanisms directed against donor endothelial and vascular antigens. Repeated subclinical injury triggers smooth muscle cell proliferation and intimal fibrosis, producing the characteristic concentric arterial narrowing seen in cardiac allograft vasculopathy (and similarly in chronic rejection of other solid organs, such as the vanishing bile duct syndrome in liver or bronchiolitis obliterans in lung). Unlike acute cellular rejection, chronic rejection is generally irreversible and does not respond well to increased immunosuppression because the pathology reflects fibrotic remodeling rather than active acute inflammation.

• A. Donor T lymphocytes attacking recipient tissue describes graft-versus-host disease, which does not occur in solid organ transplantation.
• B. Acute thrombosis from preformed antibodies describes hyperacute rejection, which occurs within minutes, not years.
• D. Calcineurin inhibitor vasospasm is a reversible drug effect, not the fibrotic vasculopathy of chronic rejection.
• E. Acute fibrinoid necrosis with immune complex deposition describes an acute, not chronic, vascular injury pattern.

Graft-versus-host disease (GVHD) occurs after allogeneic hematopoietic stem cell transplantation when mature donor T lymphocytes contained within the graft recognize recipient (host) tissue antigens as foreign and mount an immune attack against host cells. The skin, gastrointestinal tract, and liver are the classic target organs, producing a maculopapular rash (often involving palms and soles), watery diarrhea, and cholestatic jaundice, as seen in this patient roughly 2-3 weeks post-transplant. Skin biopsy characteristically shows lymphocytic infiltration at the dermal-epidermal junction with individual keratinocyte apoptosis (satellite cell necrosis). GVHD is essentially the reverse of solid organ transplant rejection: here, the immunocompetent graft attacks an immunosuppressed host rather than the host rejecting the graft.

• A. CMV reactivation can cause fever and cytopenias post-transplant but does not explain this classic triad with the described skin histology.
• C. Preformed antibody-mediated mechanisms describe hyperacute rejection in solid organ transplantation, not GVHD.
• D. This describes a rejection-type process against the graft, which is not possible here because the recipient's marrow was ablated and host immune function is suppressed.
• E. GVHD is T-cell mediated, not primarily a B-cell autoantibody process.

Cyclosporine binds cyclophilin (and tacrolimus binds FKBP), forming a complex that inhibits calcineurin, a phosphatase normally required to dephosphorylate NFAT so it can translocate to the nucleus and activate IL-2 gene transcription. By blocking this pathway, calcineurin inhibitors suppress IL-2 production and T-cell activation, which prevents allograft rejection. However, calcineurin inhibitors cause dose-dependent nephrotoxicity through afferent arteriolar vasoconstriction, producing decreased renal blood flow, elevated creatinine, and hypertension, as seen in this patient. This nephrotoxicity is a major long-term limitation of calcineurin inhibitor therapy and often requires dose adjustment or a switch to an alternative agent such as an mTOR inhibitor.

• A. IMP dehydrogenase inhibition describes mycophenolate mofetil, which causes GI upset and bone marrow suppression, not nephrotoxicity, and does not bind cyclophilin.
• C. IL-2 receptor blockade describes basiliximab, a monoclonal antibody, not a cyclophilin-binding agent.
• D. CD3 binding describes muromonab-CD3 (OKT3), an antibody used for acute rejection, not a cyclophilin-binding small molecule.
• E. mTOR inhibition describes sirolimus, which does not bind cyclophilin and is notably not nephrotoxic.

Sirolimus (rapamycin) binds FKBP, similar to tacrolimus, but the resulting complex inhibits mTOR (mammalian target of rapamycin) rather than calcineurin. mTOR inhibition blocks the T-cell response to IL-2 by preventing progression from the G1 to S phase of the cell cycle, thereby suppressing T-cell proliferation without affecting IL-2 transcription. Unlike calcineurin inhibitors, sirolimus is not nephrotoxic, making it useful in patients with calcineurin inhibitor-induced renal impairment, but its main adverse effects include pancytopenia (anemia, thrombocytopenia, leukopenia), hyperlipidemia, and impaired wound healing, as reflected by this patient's low blood counts with preserved renal function.

• B. DNA cross-linking describes alkylating agents such as cyclophosphamide, unrelated to FKBP binding or mTOR inhibition.
• C. IMP dehydrogenase inhibition describes mycophenolate mofetil, which does not bind FKBP.
• D. Calcineurin inhibition describes tacrolimus and cyclosporine, which are nephrotoxic, unlike this patient's drug.
• E. Dihydrofolate reductase inhibition describes methotrexate, which is not used in this context and does not bind FKBP.

Mycophenolate mofetil is hydrolyzed to mycophenolic acid, which reversibly inhibits inosine monophosphate dehydrogenase (IMPDH), blocking de novo guanine nucleotide synthesis. Lymphocytes depend heavily on the de novo pathway (unlike most other cells, which can use the purine salvage pathway), so this selectively suppresses T- and B-cell proliferation. However, rapidly dividing gut epithelial cells are also sensitive to guanine nucleotide depletion, producing a dose-related enterocolitis with diarrhea, abdominal pain, and crypt apoptosis resembling graft-versus-host disease histologically. Management is dose reduction or switching to an enteric-coated formulation or a different agent.

• A. Cyclosporine inhibits calcineurin and is associated with nephrotoxicity, hypertension, and gingival hyperplasia.
• B. Prednisone causes hyperglycemia, osteoporosis, and mood changes, not IMPDH-mediated GI toxicity.
• D. Tacrolimus inhibits calcineurin and causes nephrotoxicity and neurotoxicity, not this enterocolitis pattern.
• E. Sirolimus inhibits mTOR and typically causes hyperlipidemia, thrombocytopenia, and impaired wound healing.

Infliximab is a chimeric monoclonal antibody that binds and neutralizes TNF-alpha, a cytokine essential for the formation and maintenance of granulomas that wall off latent Mycobacterium tuberculosis. By inhibiting TNF-alpha, infliximab can cause breakdown of these granulomas, leading to reactivation of latent TB, which is why patients must be screened with a TB skin test or interferon-gamma release assay and chest imaging before starting anti-TNF therapy. The naming convention "-ximab" denotes a chimeric (part mouse, part human) monoclonal antibody. This adverse effect is a class effect of TNF-alpha inhibitors such as infliximab, adalimumab, and etanercept.

• B. IL-1 inhibition describes anakinra, used in autoinflammatory conditions, not TNF-mediated granuloma maintenance.
• C. IL-6 receptor blockade describes tocilizumab, which also increases infection risk but through a different cytokine pathway.
• D. Alpha-4 integrin blockade describes natalizumab, associated with PML, not TB reactivation.
• E. CD20 depletion is the mechanism of rituximab, not infliximab, and is associated with hepatitis B reactivation.

Rituximab is a chimeric monoclonal antibody directed against CD20, a surface protein expressed on pre-B and mature B lymphocytes but not on plasma cells or stem cells. Binding to CD20 induces B-cell depletion through antibody-dependent cellular cytotoxicity, complement-mediated cytotoxicity, and induction of apoptosis. Because B cells (but not T cells) are depleted, rituximab is used in CD20-positive B-cell lymphomas, rheumatoid arthritis refractory to other agents, and certain autoimmune cytopenias. A key adverse effect is hepatitis B reactivation, so screening is required before treatment.

• A. This describes a mechanism unrelated to rituximab's actual target and effect.
• C. CD52 is the target of alemtuzumab, which depletes both T and B lymphocytes.
• D. IL-2 receptor blockade describes basiliximab, used in transplant induction, not rituximab.
• E. Bruton tyrosine kinase inhibition describes ibrutinib, an oral small-molecule drug, not a monoclonal antibody.

Tocilizumab is a humanized monoclonal antibody that blocks the interleukin-6 receptor. IL-6 is the principal cytokine driving hepatocyte synthesis of acute-phase proteins, including C-reactive protein and fibrinogen, and also promotes Th17 differentiation and B-cell activation. By blocking IL-6 signaling, tocilizumab reduces inflammation in rheumatoid arthritis and is also used in cytokine release syndrome after CAR T-cell therapy. In contrast, anakinra is a recombinant IL-1 receptor antagonist used in autoinflammatory syndromes and gout flares, targeting a different cytokine axis.

• A. Anakinra blocks the IL-1 receptor, not the IL-6 receptor, and does not primarily affect acute-phase protein synthesis to the same degree.
• B. Natalizumab blocks alpha-4 integrin to prevent leukocyte adhesion and migration, unrelated to IL-6 signaling.
• C. Adalimumab is a fully human anti-TNF-alpha antibody, not an IL-6 receptor blocker.
• D. Abatacept blocks T-cell costimulation by binding CD80/CD86, unrelated to IL-6 receptor blockade.

Basiliximab is a chimeric monoclonal antibody that binds CD25, the alpha subunit of the interleukin-2 receptor, which is upregulated on T lymphocytes only after antigen activation. By blocking IL-2 from binding its receptor, basiliximab prevents IL-2-driven clonal expansion of activated T cells without causing broad lymphocyte depletion. It is used for induction immunosuppression at the time of solid organ transplantation to reduce acute rejection risk, typically given as two doses around the time of transplantation. Unlike calcineurin inhibitors, it does not affect IL-2 gene transcription, only receptor binding.

• A. IMPDH inhibition describes mycophenolate mofetil, which blocks purine synthesis, not cytokine receptor binding.
• B. Calcineurin inhibition describes tacrolimus and cyclosporine, which block IL-2 transcription, not receptor binding.
• D. B7-CD28 costimulation blockade describes belatacept, a fusion protein, not basiliximab.
• E. Lymphocyte-depleting agents like antithymocyte globulin, not basiliximab, work through complement-mediated lysis.

Natalizumab blocks alpha-4 integrin on leukocytes, preventing their adhesion to VCAM-1 and subsequent migration across the blood-brain barrier. While this reduces inflammatory demyelination in multiple sclerosis, it also impairs immune surveillance within the central nervous system, allowing reactivation of latent JC virus (a polyomavirus) with lytic infection of oligodendrocytes. This causes progressive multifocal leukoencephalopathy (PML), presenting with subacute focal neurologic deficits and non-enhancing white matter lesions on MRI. Risk is highest in patients who are JC virus antibody-positive, have had prior immunosuppressant use, and have been on natalizumab for more than 2 years.

• A. Tuberculomas are typically enhancing, ring-like mass lesions, and TB is not the mechanism linked to natalizumab.
• B. HSV encephalitis typically causes temporal lobe necrosis with hemorrhage, not the gradual demyelinating pattern described, and CSF PCR specifies JC virus.
• D. CMV ventriculoencephalitis occurs in severely immunosuppressed patients (e.g., advanced HIV) and the CSF finding specifies JC virus, not CMV.
• E. EBV-driven lymphoproliferation is associated with T-cell-depleting therapies and organ transplantation, not natalizumab's integrin blockade.

Glucocorticoids bind cytoplasmic glucocorticoid receptors, and the hormone-receptor complex translocates to the nucleus to inhibit transcription factors such as NF-kB (reducing transcription of pro-inflammatory cytokines) and to induce transcription of annexin A1, also called lipocortin-1. Annexin A1 inhibits phospholipase A2, decreasing release of arachidonic acid and downstream synthesis of prostaglandins and leukotrienes. This broad anti-inflammatory and immunosuppressive effect, combined with inhibition of NF-kB-driven cytokine transcription, underlies the therapeutic use of glucocorticoids in transplant immunosuppression and autoimmune disease.

• A. FK-binding protein 12 is the intracellular target of tacrolimus, not induced by glucocorticoids.
• B. Calcineurin is the phosphatase inhibited by cyclosporine and tacrolimus, not a product induced by glucocorticoids.
• C. Cyclophilin is the intracellular binding protein for cyclosporine, unrelated to glucocorticoid action.
• D. Inosine monophosphate dehydrogenase is inhibited by mycophenolic acid, not related to glucocorticoid mechanism.

Azathioprine is a prodrug converted to 6-mercaptopurine, which is normally inactivated by xanthine oxidase (among other enzymes) to 6-thiouric acid. Allopurinol, used for gout, is a xanthine oxidase inhibitor, and when co-administered with azathioprine it blocks this inactivation pathway, causing accumulation of toxic thioguanine nucleotide metabolites. This leads to severe, potentially life-threatening bone marrow suppression, manifesting as leukopenia and thrombocytopenia with fever and mucosal ulcers. If concurrent use is necessary, the azathioprine dose must be reduced by approximately 75%, and this interaction is a classic board-testable example of a pharmacokinetic drug interaction.

• B. Allopurinol's myelosuppressive effect in this scenario is mediated through its enzyme inhibition and the resulting drug interaction, not a direct independent marrow toxicity.
• C. Allopurinol does not induce cytochrome P450 enzymes; it is primarily a xanthine oxidase inhibitor.
• D. Protein-binding displacement is not the mechanism of the azathioprine-allopurinol interaction.
• E. Thiopurine methyltransferase inhibition describes a separate genetic polymorphism risk factor for azathioprine toxicity, not the mechanism of the allopurinol interaction, which is xanthine oxidase inhibition.

Live attenuated vaccines contain organisms that replicate transiently in host cells, so infected cells display pathogen-derived peptides on MHC class I, activating CD8+ cytotoxic T cells and producing robust cell-mediated and mucosal (IgA) immunity similar to natural infection. Inactivated vaccines cannot replicate and are processed mainly through the exogenous (MHC class II) pathway, favoring humoral antibody responses with weaker CD8+ T-cell activation. This replication capacity is why live vaccines generally produce more durable, broader immunity but carry a risk of causing disease in immunocompromised hosts.

• B. A polysaccharide-only response describes unconjugated polysaccharide vaccines, not live attenuated vaccines.
• C. Vaccine-induced immunity relies primarily on adaptive B and T cell responses, not isolated complement pathway activation.
• D. Short-lived responses requiring frequent boosters are more typical of inactivated/subunit vaccines, not live attenuated ones.
• E. Both vaccine types eventually induce class switching with helper T cell support, not isolated IgM.

Pure polysaccharide antigens are T-independent and cannot be presented on MHC class II, so they poorly activate CD4+ helper T cells, resulting in weak, short-lived IgM responses without memory formation, especially in children under 2 years whose T-independent responses are immature. Conjugating the polysaccharide to a protein carrier allows the carrier peptide to be processed and presented on MHC class II, recruiting CD4+ helper T cells that provide cognate help to B cells, driving germinal center formation, class switching, affinity maturation, and immunologic memory. This is the basis for conjugate vaccines such as Hib, pneumococcal (PCV13), and meningococcal conjugate vaccines.

• A. Long-lived plasma cells require T cell help and germinal center reactions, which depend on antigen-presenting cells.
• B. Conjugate vaccines specifically promote germinal center formation, unlike marginal zone-only responses to pure polysaccharide.
• C. The benefit of conjugation is T cell help for antibody responses, not complement activation independent of antibody.
• D. This describes the T-independent mechanism of the unconjugated polysaccharide, which conjugation is meant to overcome.

The varicella vaccine contains a live attenuated virus that normally undergoes limited replication controlled by an intact immune system. In patients on immunosuppressive therapy, such as high-dose corticosteroids (prednisone >=20 mg/day for >=2 weeks) or mycophenolate mofetil, this controlled replication can become uncontrolled, leading to disseminated, potentially fatal varicella infection. Live vaccines are therefore contraindicated in significantly immunosuppressed patients, who should instead receive inactivated vaccines when available or wait until immunosuppression is reduced.

• A. The contraindication relates to the live virus component, not an adjuvant effect on lupus activity.
• B. The concern with live vaccines in immunosuppression is uncontrolled viral replication, not immune complex-mediated type III hypersensitivity.
• D. Corticosteroids transiently impair but do not permanently destroy germinal center function.
• E. Vaccine immunogenicity declines with heavier immunosuppression but is not categorically absent in all adults; the primary safety issue is live virus replication risk.

Rabies immune globulin provides passive immunity: preformed, pathogen-specific antibodies transferred directly to the patient that act immediately to neutralize virus at the wound site, which is critical because rabies has a long but ultimately fatal incubation period and active immunity from vaccination takes 1-2 weeks to develop. Passive immunity provides immediate but short-lived protection (weeks) since no memory cells are generated, whereas the concurrently administered vaccine induces active immunity with slower onset but long-lasting memory. This combination bridges the gap between exposure and the development of the patient's own protective antibody response.

• B. Immune globulin administration does not require or enhance complement-mediated germinal center reactions.
• C. Immune globulin contains preformed antibodies; it does not stimulate the recipient's own memory B cells since no prior immunization has occurred.
• D. Passive immunoglobulin does not activate the recipient's T cells; it acts via direct antibody neutralization.
• E. Passive immunity is short-lived (weeks) because no memory cells are formed; long-term memory comes from the active vaccine component.

The Fab (fragment antigen-binding) region is composed of the variable domains of both the heavy and light chains (along with the first constant domain of each), and it is this hypervariable region that directly contacts and binds antigen with high specificity. The Fc (fragment crystallizable) region, made of the constant heavy chain domains, mediates effector functions, including binding to Fc receptors on phagocytes and NK cells and activating the classical complement pathway, but it does not participate in antigen recognition. This structural division allows antibodies to perform both antigen-specific recognition (Fab) and effector cell/complement engagement (Fc) independently.

• A. Glycosylation of the Fc region modulates effector function and antibody half-life but is not involved in antigen recognition.
• B. The hinge region provides flexibility between Fab arms and the Fc stem but does not itself bind antigen.
• D. The constant heavy chain region alone forms part of the Fc fragment, which mediates effector functions, not antigen binding.
• E. The J chain helps link IgM pentamers or IgA dimers together and is unrelated to antigen-binding specificity.

Activation-induced cytidine deaminase (AID) deaminates cytosine to uracil within immunoglobulin variable region genes in germinal center B cells, and subsequent error-prone repair introduces point mutations, a process called somatic hypermutation. B cells bearing mutated receptors with higher antigen affinity are then preferentially selected by follicular dendritic cells and T follicular helper cells, a process called affinity maturation, which underlies the increased antibody affinity seen in secondary immune responses. AID also mediates class switch recombination, and its deficiency causes hyper-IgM syndrome type 2 with elevated IgM and absent other isotypes.

• A. DNA topoisomerase II relieves supercoiling during replication and is not specific to immunoglobulin gene diversification.
• B. RAG1/RAG2 mediate V(D)J recombination during early B and T cell development in the bone marrow/thymus, generating initial receptor diversity, not somatic hypermutation.
• C. TdT adds random nucleotides at junctions during V(D)J recombination, contributing to junctional diversity, not affinity maturation in germinal centers.
• E. Telomerase maintains chromosome ends and is unrelated to antibody gene mutation or affinity maturation.

RAG1 and RAG2 recognize recombination signal sequences flanking V, D, and J gene segments and introduce double-strand DNA breaks that initiate V(D)J recombination, a process essential for generating diverse antigen receptors in both developing B cells (immunoglobulin genes) and T cells (T cell receptor genes). Because RAG enzymes are required in both lineages, their deficiency causes a form of severe combined immunodeficiency (SCID) with absent mature B and T lymphocytes but normal NK cell numbers. This differs from Bruton tyrosine kinase deficiency (X-linked agammaglobulinemia), which selectively blocks B cell maturation, or adenosine deaminase deficiency, which causes SCID through toxic metabolite accumulation rather than failed receptor gene rearrangement.

• A. BTK deficiency selectively impairs B cell maturation (X-linked agammaglobulinemia) and does not affect T cell development.
• C. AID acts later, in mature germinal center B cells, mediating somatic hypermutation and class switching, not initial V(D)J recombination.
• D. Nonhomologous end joining repairs the breaks RAG creates but does not initiate recognition or cleavage at recombination signal sequences; isolated deficiency of this repair step causes a related but distinct radiosensitive SCID phenotype.
• E. ADA deficiency causes SCID through accumulation of toxic metabolites that kill lymphocytes, not through failure of V(D)J recombination itself.

NK cells kill target cells through two main mechanisms: recognition of absent or reduced MHC class I ("missing self"), which removes the inhibitory signal normally delivered through killer immunoglobulin-like receptors (KIRs), and antibody-dependent cellular cytotoxicity (ADCC), in which CD16 (FcγRIII) on the NK cell surface binds the Fc region of IgG antibodies coating a target cell, triggering NK cell degranulation and target cell lysis via perforin and granzymes. This dual mechanism allows NK cells to kill virus-infected or tumor cells that evade T cell recognition by downregulating MHC I, as well as antibody-opsonized cells. CMV and other viruses that downregulate MHC I to evade cytotoxic T lymphocytes paradoxically become more susceptible to NK cell-mediated missing-self killing.

• A. CD8 is a coreceptor on cytotoxic T lymphocytes that binds MHC class I, not an Fc receptor on NK cells.
• C. CD3 is part of the T cell receptor complex and is characteristically absent on NK cells, distinguishing them from T cells.
• D. CD28 is a T cell costimulatory receptor that binds B7 on antigen-presenting cells and is not involved in NK cell Fc receptor-mediated killing.
• E. KIRs deliver inhibitory (or activating) signals based on MHC class I recognition, underlying missing-self killing, but do not bind antibody Fc regions.