Origin and Differentiation
Both populations originate in the bone marrow from a common myeloid progenitor. Further cell line separation occurs in the spleen.
- Basophils mature in the bone marrow or spleen and enter the bloodstream as fully mature cells ready to perform their functions.
- Mast cells enter the blood as immature precursors (iMCs). From the bloodstream, they head into tissues—the largest number settles in the intestinal mucosa, where their final maturation takes place.
The main stimulators of mast cell development are IL-3 (CD123 receptor) and stem cell factor SCF (its receptor is CD117 or c-Kit). For cell maturation directly in the mucous membranes, the tissue-specific factor IL-33 is critically important.
Cytoplasmic granules and their contents
The main morphological feature of mastocytes and basophils is the abundance of granules in the cytoplasm (a mast cell can contain from 10 to 150). They store pre-synthesized (preformed) substances:
- Vasoactive amines: the main one is histamine, which mediates most of the effects in allergic reactions.
- Proteases: tryptases, chymases, as well as metalloproteinase (carboxypeptidase A).
- Carbohydrate components: heparin or chondroitin sulfates.
- Enzymes: peroxidase, RNase, and others.
The release of this arsenal (degranulation) occurs when FcεRI receptors on the cell membrane cross-link with the IgE-antibody + allergen complex. An alternative activation pathway is an increase in intracellular calcium or cAMP.
Synthesized mediators and functions
In addition to releasing pre-formed granules, stimulated mast cells begin actively synthesizing new molecules. Eicosanoids are produced in huge quantities—primarily prostaglandin E2 and leukotriene C4.
Mastocytes secrete a wide spectrum of cytokines whose profile resembles type 2 T-helper cells (Th2). Some mediators, such as IL-4, TNF, and GM-CSF, are synthesized spontaneously. The continuous secretion of IL-4 allows mast cells to participate in the induction of Th2 cell differentiation. Furthermore, the presence of MHC-I, MHC-II molecules, and the CD86 co-stimulatory molecule on their surface enables mastocytes to function as antigen-presenting cells (APCs).
Differences between basophils and mast cells
Basophils perform a similar function—they support allergic inflammation, but have a number of fundamental differences:
- Localization and migration: Normally found only in the bloodstream (up to 0.5% of all leukocytes). They require time to migrate into tissue to participate in a reaction, so they do not initiate the earliest stages of the response.
- Receptors: Basophils are richer in chemotactic receptors (for anaphylatoxins C3a/C5a, chemokines), but completely lack c-Kit and FcRγI receptors, and their spectrum of Toll-like receptors is significantly poorer.
- Secretion: Basophils have fewer granules, a more meager protease composition, and a limited spectrum of synthesized substances (mainly IL-4, IL-13, leukotriene C4).
- Regeneration: Unlike mastocytes, basophils do not regenerate their granules after degranulation.
Two human mast cell populations
Depending on the microenvironment and localization, mastocytes differentiate into two types that differ in functions and enzymatic composition:
| Property | Mucosal | Serous |
|---|---|---|
| Localization | Intestinal mucosa, respiratory tract | Dermis, serous cavities, tonsils |
| Main protease | Tryptase | Tryptase, chymase, cathepsin G |
| Proteoglycan | Chondroitin sulfate | Heparin |
| Thymus dependence | Present | No |
| Lifespan | Less than 40 days | More than 40 days |
| Association with pathology | Allergies, defense against parasites | Sclerosis processes |
Briefly: answers to topical questions
What receptors are present on the mast cell membrane?
Mast cell membranes feature receptors for immunoglobulins, complement components, and pattern recognition receptors.
- FcεRI — high-affinity IgE receptors (key for function execution).
- FcγI receptors — high-affinity IgG receptors.
- CD117 (c-Kit) — receptor for stem cell factor (SCF).
- CD123 — receptor for interleukin-3 (IL-3).
- Complement receptors — receptors for C3b and C3d; C3aR for the C3a anaphylatoxin is also present. Mucosal mast cells lack the CR1 receptor.
- TLRs — pattern recognition receptors TLR-2, TLR-3, and TLR-4.
Which lipid mediators and cytokines are synthesized de novo by mast cells?
Upon stimulation, mast cells synthesize lipid mediators and a wide range of cytokines de novo.
- Lipid mediators — eicosanoids, most abundantly leukotriene C4 and prostaglandin E2, as well as platelet-activating factor (PAF).
- Th2-profile cytokines — IL-3, IL-4, IL-5, IL-6, IL-10, IL-13, GM-CSF.
- Pro-inflammatory cytokines — IL-1, IL-8, IL-12, IL-18, IL-21, IL-23, TNFα.
- Homeostatic cytokines — IL-7, IL-15, TGFβ, certain chemokines, and major types of interferons.
- Peptide growth factors — VEGF, FGF, NGF.
Mast cells spontaneously produce IL-4, TNF, and GM-CSF.
Which CD markers are used for the immunophenotyping of mature basophils?
The following CD markers and receptors are used to characterize mature basophils:
- CD11b is a marker of mature basophils.
- CD23 (FcεRII) is a low-affinity IgE receptor.
- CD63 and CD203c are basophil activation markers used in in vitro functional tests.
Unlike mast cells, basophils do not express CD117 (c-Kit).
What is the intracellular signaling cascade during mast cell activation via the FcεRI receptor?
The intracellular signaling cascade upon FcεRI activation begins with the phosphorylation of ITAM motifs in the receptor's γ-chain and signal transduction to the Syk tyrosine kinase. The pathway then diverges into three branches:
- Degranulation — Syk activates phospholipase C (PLCγ), which cleaves PIP2 into DAG and inositol trisphosphate. This increases Ca²⁺ levels, activates protein kinase C, and triggers granule exocytosis.
- Eicosanoid synthesis — Syk activates phospholipase A2 (PLA2), releasing arachidonic acid for the production of prostaglandins and leukotrienes.
- Cytokine synthesis — Syk activates the Ras protein, triggering the MAP kinase cascade. This activates transcription factors (Fyn/Jun) and the expression of cytokine genes.
Through which mechanisms do mast cells participate in the immune response against helminths?
Mast cells participate in the anti-helminth immune response by initiating inflammation and recruiting effector cells.
- Secretion of cytokines and mediators — stimulated by IL-4 from Th2 lymphocytes, mast cells release IL-4, IL-5, TNFα, neutrophil chemotactic factor, and PAF.
- Effector recruitment — mast cell mediators recruit eosinophils, basophils, and neutrophils to the site of infection.
- Effector action of eosinophils — recruited eosinophils form a protective wall around the helminth and release toxic proteins MBP and ECP, causing parasite death.
To remember the main mast cell growth factor, use this mnemonic: mast cells are large cells ("like a whale" [kit in Russian]). Their key stem cell factor receptor is appropriately named c-Kit (CD117).
Class and exam questions
Why do mast cells trigger allergies faster than basophils?
Mast cells are permanently stationed in skin and mucosal tissues, making them the first to encounter allergens. Basophils, on the other hand, reside in the blood and require additional time to migrate to the site of inflammation.
What happens to a mast cell after degranulation?
Unlike many other cells of the immune system, the mast cell does not die. It remains viable, and the expelled cytoplasmic granules eventually regenerate.
Can mast cells recognize bacteria directly?
Yes, pattern-recognition receptors (TLR-2, TLR-3, TLR-4) are present on their surface. This allows them to respond directly to pathogens and their products.
What else to study on this topic
- The membrane phenotype of mast cell precursors at various stages of migration (β7, CXCR2 markers).
- Mechanisms of antigen presentation by mast cells (the role of MHC-II and CD86).
- The spectrum of proteases: differences between tryptases, chymases, and metalloproteinases.
- The role of IL-33 as a tissue-specific maturation factor in mucosal membranes.
- Immunoregulatory function and induction of Th2 lymphocytes via spontaneous IL-4 production.
Master the entire topic in the "Immunology" course
This is a summary. The course covers everything tested in classes and exams.
- Comprehensive study guide on "Mast Cells and Basophils" and 723 other pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 23 video lessons on the subject
Other topics in the "Innate Immunity" section
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.