Structure of the B-cell receptor
The BCR complex consists of two main components:
- Antigen-recognizing part proper: membrane immunoglobulin (mIg). It has a structure analogous to soluble antibodies (4 chains: two heavy H and two light L chains), but differs by having a hydrophobic transmembrane segment for anchorage in the membrane. The cytoplasmic tail of mIg is very short (3 to 28 amino acids). Unlike serum IgM (a pentamer), membrane IgM always exists as a 7S monomer.
- Signaling part: Igα and Igβ heterodimers (also known as CD79a and CD79b). They are non-covalently associated with mIg and are responsible for intracellular signal transduction through their connection with intracellular protein tyrosine kinases.
Coreceptor complex
For full activation of a B lymphocyte and signal transduction, functionally associated molecules that physically bind to the BCR only at the moment of cell activation are incorporated into the receptor apparatus:
- CD19: a member of the immunoglobulin superfamily. Its cytoplasmic domain interacts with the lipid kinase PI3K, playing a crucial signaling role.
- CD21 (CR2): a receptor for complement components. It enhances the antigenic signal and regulates B-cell activity.
- CD81: a protein from the tetraspanin family that spans the membrane four times. Its exact function has not yet been definitively established.
Intracellular signaling
Antigen binding to the BCR triggers a cascade of reactions involving multiple kinases:
- The tyrosine kinase Fyn is directly associated with the cytoplasmic tails of mIg.
- The tyrosine kinases Blk, Lyn, Lck, Syk, as well as Src, are associated with CD79, CD19, and CD81 molecules and transmit the activation signal downstream.
- The lipid kinase PI3K is located near the cytoplasmic tail of CD19.
Differences between BCR and T-cell receptor (TCR)
Despite sharing a common function—antigen recognition—B- and T-cell receptors have several fundamental differences.
| Characteristic | B-cell receptor (BCR) | T-cell receptor (TCR) |
|---|---|---|
| Recognizing molecule | Immunoglobulin (4 chains: H and L) | TCR (2 chains: α and β or γ and δ) |
| Associated molecules | Igα/Igβ dimers | CD3 complex and ζ2/ζη dimer |
| Secreted forms | Present (antibodies) | No |
| Isotype switching | Present | No |
| Somatic mutagenesis | Present | No |
Briefly: answers to topical questions
How does intracellular signal transduction occur upon B-cell receptor activation?
Intracellular signal transduction upon B-cell receptor activation is initiated by Src family tyrosine kinases (Lyn, Fyn, Blk), which activate the kinase Syk. Antigen binding and receptor aggregation induce conformational changes that are transmitted to the Igα/Igβ dimer. Next, the signal is relayed via adapter proteins (Vav, Sos, Grb2) to major intracellular cascades:
- Phospholipase C cascade (PLCγ) — leads to an increase in intracellular Ca2+ and activation of the NFAT factor.
- Ras-MAPK pathway (Ras-MAPK) — leads to the synthesis of c-Fos.
- Rac/Rho pathways (Rac/Rho) — activate c-Jun and p38.
- Protein kinase C pathway (PKC) — releases the NFκB factor.
The end result is the expression of B-cell activation genes. Additional signal amplification occurs with the participation of the coreceptor complex (CD21, CD19).
What domains make up the heavy and light chains of membrane immunoglobulin?
The heavy and light chains of membrane immunoglobulin consist of variable (V) and constant (C) domains. Variable domains are located at the N-termini of the polypeptide chains and are responsible for antigen recognition, while constant domains form their backbone.
| Characteristic | Heavy chains (H chains) | Light chains (L chains) |
|---|---|---|
| Number of V domains | 1 | 1 |
| Number of C domains | 3–4 | 1 |
What types of antigen epitopes can the B-cell receptor recognize?
The B-cell receptor is capable of recognizing almost all possible molecular configurations (epitopes). It interacts with antigens in their native, uncleaved form.
- Free antigens (Ag) — molecules in dissolved form (e.g., delivered by afferent lymph).
- Membrane-bound antigens (Ag) — molecules presented on the surface of other cells.
Effective recognition and signal initiation often require the presence of repeating epitopes within the antigen molecule, which induces cross-linking (aggregation) of receptors.
The B-cell coreceptor complex is easy to remember by the numbers ending in "1": CD19, CD21, CD81.
Class and exam questions
What is the difference between serum and membrane IgM?
Membrane IgM is a 7S monomer, whereas serum (secreted) IgM exists as a 19S pentamer.
Which molecules mediate signal transduction from the BCR into the cell?
The main role in signal transduction is played by Igα and Igβ (CD79) molecules, as well as the CD19 coreceptor, associated with various intracellular kinases (Fyn, Syk, PI3K, etc.).
Does somatic mutagenesis occur in the B-cell receptor?
Yes, somatic mutagenesis is characteristic of the BCR during the immune response, unlike the T-cell receptor, in which it is absent.
What else to study on this topic
- Generation of the antigen-recognizing repertoire of B lymphocytes
- Mechanism of immunoglobulin C-domain isotype switching
- Role of Blk, Lyn, Lck, and Syk kinases in the activation cascade
- History of T-cell receptor discovery and hybridoma technology
Master the entire topic in the "Immunology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on «B-cell receptor» and 723 more pages on 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 "Adaptive Immunity" section
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.