Stages of the response and the first line of defense
The dynamics of the immune response is divided into two key stages: the inductive and effector phases. The outcome of a successful fight is a state of immunity, which ensures resistance to a specific agent. Resistance is maintained by surviving effector cells, while long-term protection is provided by immunological memory cells.
In the inductive phase, exclusively innate immunity factors operate. They are mobilized immediately after pathogen entry. Everything begins with the recognition of foreign molecules (PAMPs) via membrane TLRs (Toll-like receptors). This leads to cell activation and the secretion of pro-inflammatory cytokines. Cytokines recruit leukocytes to the focus — neutrophils arrive first, followed by monocytes.
The primary reaction at this stage is phagocytosis. To facilitate it, opsonization (tagging) of pathogens occurs using acute-phase proteins, such as pentraxins. Early defense also involves:
- The complement system (predominantly via the alternative pathway).
- Natural IgM antibodies synthesized by B1 lymphocytes.
- Early cytokines: IFNγ (from NKT cells) and IFNα (from plasmacytoid dendritic cells).
- Bactericidal factors (defensins) and non-immune cells (epithelium, endothelium) mimicking macrophage effects.
Features of the antiviral response
Defense against viruses differs cardinally from combating large agents (bacteria or fungi). Inflammation proceeds with significantly less involvement of phagocytes. Main viral patterns are recognized by intracellular TLRs; therefore, phagocytosis is primarily needed for clearing already infected and apoptotic cells.
The earliest and most critical mechanism is the production of type I interferons (IFNα and IFNβ), which exert a direct antiviral effect. Their main source is plasmacytoid dendritic cells. The primary front-line fighters here are natural killer cells (NK cells). They recognize stress molecules on the surface of infected cells and destroy them, receiving additional stimulation from NKT cells.
Formation of adaptive immunity and effector phase
Concurrently with innate defense, preparation for the specific strike takes place. Myeloid dendritic cells act as initiators: they capture pathogen fragments and transport them to lymph nodes to present them to CD4+ and CD8+ T lymphocytes. Proliferation of single, strictly specific clones is triggered.
In the effector phase (late first to early second week), innate mechanisms are potently amplified by adaptive factors. Weapon selection depends on the enemy's localization:
- Intracellular pathogens in the cytoplasm (viruses, chlamydia, listeria). Presented via MHC-I. Main effectors are cytotoxic T lymphocytes (CTLs), which kill the infected cell.
- Intracellular pathogens in vesicles (mycobacteria, leishmania). Destroyed via intracellular digestion following macrophage activation by signals from Th1 helpers.
- Extracellular pathogens and toxins (streptococci, fungi). Destroyed by the humoral response: antibodies neutralize toxins and opsonize bacteria (switching from IgM to IgG). The complement system enhances opsonization via C3b and induces lysis.
A special front is mucosal defense. Here, secretory IgA plays the leading role, capable of functioning even outside the internal environment (e.g., in the intestinal lumen during helminth invasions).
Termination of the response and immune-mediated damage
Once the pathogen is eliminated, the immune system must downregulate. Immunoregulation mechanisms are engaged: Fc-dependent suppression of the humoral response and the activity of regulatory T cells, which limit lymphocyte activity. Consequently, only specific memory cells remain in the body.
However, the powerful arsenal of immunity can cause immune-mediated damage to self tissues:
- If the pathogen is difficult to eradicate (as in tuberculosis), unbalanced hypersensitivity develops, and granulomas form to isolate the threat.
- In viral infections (e.g., hepatitis), tissues are often damaged not by the virus itself, but by an excess of cytokines, the release of enzymes from necrotic cells, and the reaction to alarmins (danger signals from damaged tissues).
- Sometimes antibodies and lymphocytes begin to cross-react with self tissues, triggering autoimmune processes.
Briefly: answers to topical questions
Which immunoglobulin classes participate in the effector phase of the immune response, and what are their specific functions?
During the effector phase, antibodies that enhance innate immunity reactions are involved.
- IgG — ensure pathogen opsonization, enhancing phagocytosis; participate in complement activation, leading to additional opsonization and direct cytolysis.
- Secretory IgA — serves as the main humoral defense factor of mucous membranes and is capable of acting outside the body's internal environment.
- Switching antibody synthesis from IgM to IgG increases opsonization efficiency. Natural IgM participate in early defense and serve as an additional stimulus for complement activation.
What immunoregulation mechanisms ensure the downregulation of the immune response after pathogen eradication?
Downregulation of the immune response following pathogen eradication is ensured by active immunoregulation mechanisms:
- Fc-dependent suppression of the humoral immune response.
- The activity of regulatory T cells — both natural and adaptive.
The result is the restriction of all types of lymphocyte activity.
Which families of pattern-recognition receptors (PRRs), besides TLRs, participate in PAMP recognition?
Besides TLRs, several other families of pattern-recognition receptors (PRRs) participate in recognizing pathogen-associated molecular patterns (PAMPs).
- NLRs (NOD-like receptors) — alongside TLRs, they are major factors in myeloid cell activation (including cytosolic NALP receptors involved in inflammasome formation).
- C-type lectin receptors — for example, Dectin-1, which binds yeast β-glucans and induces cytokine genes.
- Integrins — act as coreceptors capable of activating transcription factors NF-κB and AP-1.
What are the pathways of complement system activation and what are their key differences?
There are three pathways of complement system activation that differ in their initiation mechanisms but share an identical cell lysis phase.
| Activation pathway | Initiation (trigger) | Formed C3 convertase |
|---|---|---|
| Classical | Antigen-antibody complex (Ag-Ab) binds C1q | C4b2b |
| Lectin | Carbohydrates on the pathogen surface bind MBL or ficolins | C4b2b |
| Alternative | Spontaneous hydrolysis of C3 to C3(H2O) | C3bBb |
Class and exam questions
Which cells trigger the adaptive immune response?
The main initiators are myeloid dendritic cells. They capture the antigen, migrate to lymph nodes, and present it to T lymphocytes.
What is the difference between the function of NK cells and cytotoxic T lymphocytes (CTL)?
NK cells belong to innate immunity and respond to stress molecules on infected cells. CTLs are adaptive immunity factors possessing strict antigenic specificity for a specific pathogen.
What is the role of Th1 cells in the effector phase?
Th1 helpers secrete cytokines (specifically IFNγ) that strongly stimulate macrophage activity, shifting phagocytosis into an "overdrive" mode to destroy intracellular pathogens in vesicles.
What else to study on this topic
- Mechanisms of PAMP recognition via Toll-like receptors (TLRs).
- The process of antigen presentation via MHC-I and MHC-II molecules.
- The role of regulatory T cells (Treg) in suppressing the immune response.
- Pathogenesis of granuloma formation in persistent infections.
- Mechanisms of antibody isotype switching from IgM to IgG and IgA.
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