B cell receptor

B Cell Receptor

The B cell receptor (BCR) is a complex of proteins on the surface of B cells that allows these cells to recognize specific antigens. The BCR plays a crucial role in the adaptive immune response by enabling B cells to detect and respond to pathogens.

Structure

The B cell receptor is composed of two main parts:

• The membrane-bound immunoglobulin (Ig) molecule, which is responsible for antigen binding.
• The signal transduction component, which includes the Igα (CD79a) and Igβ (CD79b) proteins.

Immunoglobulin

The immunoglobulin component of the BCR is a membrane-bound form of the antibody that the B cell is programmed to produce. It consists of two heavy chains and two light chains, forming a Y-shaped structure. The variable regions of the heavy and light chains form the antigen-binding site, which is highly specific to a particular antigen.

Signal Transduction

The Igα and Igβ proteins are non-covalently associated with the membrane-bound immunoglobulin. These proteins contain immunoreceptor tyrosine-based activation motifs (ITAMs) in their cytoplasmic tails, which are crucial for initiating intracellular signaling upon antigen binding.

Function

The primary function of the B cell receptor is to recognize and bind to specific antigens. Upon binding to its specific antigen, the BCR undergoes a conformational change that initiates a cascade of intracellular signaling events. This signaling leads to several outcomes:

• Activation of the B cell, leading to proliferation and differentiation into plasma cells and memory B cells.
• Internalization of the antigen-BCR complex, followed by antigen processing and presentation to T helper cells via MHC class II molecules.
• Production of antibodies that are secreted into the bloodstream to neutralize the pathogen.

BCR Signaling Pathway

The signaling pathway initiated by the BCR involves several key molecules and steps:

1. Antigen Binding: The BCR binds to its specific antigen. 2. Phosphorylation of ITAMs: The ITAMs on Igα and Igβ are phosphorylated by Src-family kinases such as Lyn. 3. Recruitment of Syk: The phosphorylated ITAMs recruit and activate the Syk kinase. 4. Activation of Downstream Pathways: Syk activation leads to the activation of multiple downstream signaling pathways, including the PI3K/Akt pathway, the MAPK pathway, and the NF-κB pathway. 5. Gene Transcription: These pathways culminate in the activation of transcription factors that drive the expression of genes necessary for B cell activation and differentiation.

Clinical Significance

Dysregulation of BCR signaling can lead to various diseases, including autoimmune diseases and B cell lymphomas. Targeting BCR signaling pathways is a therapeutic strategy in treating certain types of B cell malignancies.

See Also

• T cell receptor
• Antibody
• Adaptive immune system

References

• Janeway, C. A., et al. (2001). Immunobiology: The Immune System in Health and Disease. 5th edition. New York: Garland Science.
• Alberts, B., et al. (2002). Molecular Biology of the Cell. 4th edition. New York: Garland Science.