B-Cell Receptor Complexes

B-cell receptors (BCRs) are membrane-bound immunoglobulin molecules found on the surface of B lymphocytes. They are responsible for recognizing and binding to specific antigens, initiating the B cell immune response. BCRs have a similar overall structure to antibody molecules secreted by plasma cells.

A BCR is composed of two heavy chains and two light chains, held together by disulfide bonds in a Y-shaped structure.

Heavy chain:

• Variable (V) domain – Contains hypervariable complementarity determining regions (CDRs) that bind to the antigen
• Constant (C) domains – Provide structural support (CH1, CH2, CH3)
• Hinge region – Flexible region between V and C domains
• Transmembrane domain – Anchors the BCR in the B cell membrane
• Short cytoplasmic tail

Light chain:

• Variable (V) domain with CDRs
• Constant (C) domain

The heavy and light chain variable domains come together to form the antigen-binding site.

CD79 Co-Receptor Molecules:

Unlike antibodies, BCRs do not have an Fc region but instead associate with two co-receptor molecules, CD79a (Igα) and CD79b (Igβ). These are disulfide-linked heterodimers that provide the signaling component:

• CD79a/b have an extracellular domain that non-covalently associates with the BCR
• A transmembrane domain
• An immunoreceptor tyrosine-based activation motif (ITAM) in the cytoplasmic tail

When an antigen binds the BCR, the CD79a/b ITAMs become phosphorylated, initiating downstream signaling cascades that activate the B cell.

So in summary, the BCR monomer consists of the membrane-bound immunoglobulin (heavy + light chains) associated with the CD79a/b signaling subunits. Multiple BCR monomers can oligomerize on the surface, enabling antigen binding and B cell activation via the CD79 ITAM signaling domains.

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Source: Claude 3 Sonnet response prompted and edited by Joel Graff.

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