Immunology · Cytokines and molecules
Fc receptors, BCR and TCR !
- Molecule
The BCR and TCR let B and T cells recognise one specific antigen; Fc receptors let other cells grab antibodies that are already bound to a target.
Updated 2026-11-01
What it is
The B-cell receptor (BCR) is a membrane antibody (IgM and IgD on naive B cells) paired with the signalling chains Igα/Igβ (CD79a/CD79b). The T-cell receptor (TCR, usually αβ) recognises a peptide only when presented on MHC and signals through the CD3 complex. Fc receptors bind the constant (Fc) part of antibodies and connect antibody specificity to effector cells such as macrophages, NK cells and mast cells.
Structure
- Activating receptors (BCR, TCR-CD3, FcγRI, FcγRIIa, FcγRIIIa, FcεRI) carry ITAM motifs that kinases phosphorylate.
- FcγRIIb carries an ITIM, which recruits phosphatases (SHP-1, SHIP-1) and switches signalling off.
- Each B or T cell expresses receptors of a single specificity; diversity comes from gene rearrangement of the variable regions.
The main antigen and Fc receptors
| Receptor | Cell | Ligand | Result |
|---|---|---|---|
| BCR (IgM/IgD + CD79a/b) | B cell | Native antigen | B-cell activation, plasma cells, antibodies |
| TCR αβ + CD3 | T cell (CD4 or CD8) | Peptide on MHC II or MHC I | Help (CD4) or killing (CD8) |
| FcγRI (CD64) | Macrophages, neutrophils | IgG | Phagocytosis of opsonised microbes |
| FcγRIIIa (CD16) | NK cells, macrophages | IgG on a target cell | ADCC |
| FcγRIIb (CD32b) | B cells, myeloid cells | IgG | Inhibition (ITIM) |
| FcεRI | Mast cells, basophils | IgE | Degranulation: histamine, leukotrienes |
| FcαRI (CD89) | Neutrophils, monocytes | IgA | Phagocytosis, inflammation |
| FcRn | Placenta, endothelium | IgG | Long IgG half-life, transfer to the fetus |
Clinical notes
- Many therapeutic monoclonal antibodies kill target cells through ADCC via CD16 on NK cells.
- Intravenous immunoglobulin acts partly by blocking activating Fc receptors and saturating FcRn, which speeds clearance of autoantibodies.
- Maternal IgG crosses the placenta through FcRn: protects the newborn but can also transfer autoantibodies (neonatal lupus, neonatal Graves).
- CD79a/b and CD3 are standard markers for B- and T-cell lymphomas in pathology.
Test yourself
Think of the answer, then tap the question to see it.
Which Fc receptor do NK cells use for ADCC?
FcγRIIIa (CD16), which binds IgG coating the target cell.
How does antigen recognition by the BCR differ from the TCR?
The BCR binds native antigen directly; the TCR only recognises processed peptide presented on MHC molecules.
Which receptor explains the long half-life of IgG and its passage across the placenta?
The neonatal Fc receptor (FcRn), which rescues IgG from degradation and transports it.
Related
- Interleukin-2
- IL-4, IL-5 and IL-13 (the Th2 cytokines)
- Adhesion molecules: selectins and integrins
- B cell and plasma cell
- Antibody structure and isotypes (IgG, IgA, IgM, IgE, IgD)
- MHC/HLA class I and II
- Antigen processing and presentation
- T-cell activation: two signals and superantigens
- Natural killer cell
- Type I hypersensitivity (immediate, IgE)
Related conditions
Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given.
Sources: Understanding Fc Receptor Involvement in Inflammatory Diseases: From Mechanisms to New Therapeutic Tools (Frontiers in Immunology, 2019); Molecular mechanisms regulating T-cell receptor signaling and T-cell function (Frontiers in Immunology, 2026); 21.4 The Adaptive Immune Response: B-lymphocytes and Antibodies - Anatomy and Physiology 2e | OpenStax (OpenStax, 2022); 21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types - Anatomy and Physiology 2e | OpenStax (OpenStax, 2022).
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Representative model, for educational purposes only.