B cell receptors are crucial for immune response. They consist of membrane-bound immunoglobulins and signaling components that recognize antigens and trigger activation. Understanding their structure and function is key to grasping how B cells defend against pathogens.

When B cell receptors bind antigens, they kick off complex signaling cascades. These involve kinase activation, second messenger generation, and transcription factor activation. Co-receptors fine-tune the response, either enhancing or dampening signaling to control B cell activation.

B Cell Receptor Structure

Structure of B cell receptor complex

Membrane-bound immunoglobulin (mIg) consists of two heavy chains and two light chains forming antigen-binding sites in variable regions
Signaling components include Igα (CD79a) and Igβ (CD79b) heterodimer with cytoplasmic tails containing ITAM motifs crucial for signal transduction
Accessory molecules CD19, CD21 (complement receptor 2), and CD81 enhance BCR signaling and lower activation threshold

B cell receptor in antigen recognition

Variable regions of mIg bind specific epitopes enabling clonal selection of B cells with matching receptors
Antigen binding triggers conformational changes initiating intracellular signaling cascades
Receptor-mediated endocytosis internalizes bound antigens for processing and presentation on MHC class II molecules

B Cell Receptor Signaling

Signaling cascades of B cell activation

Tyrosine kinase activation: Lyn kinase phosphorylates ITAM motifs, recruiting and activating Syk kinase
Signaling complex formation: BLNK (SLP-65) acts as scaffold protein recruiting Btk and PLCγ2
Second messenger generation: PLCγ2 cleaves PIP2 into IP3 and DAG, IP3 triggers calcium release from endoplasmic reticulum
Transcription factor activation: NF-κB pathway activated by PKC, NFAT activated through calcium-calcineurin pathway, ERK/MAPK pathway activated
Cellular responses: Proliferation, differentiation, cytokine production, and antibody class switching initiated

Co-receptors in B cell signaling

CD19-CD21-CD81 complex lowers activation threshold and enhances BCR signaling through PI3K pathway
CD45 dephosphorylates inhibitory tyrosine residues on Src-family kinases promoting BCR signaling initiation
CD22 negatively regulates BCR signaling by recruiting SHP-1 phosphatase to dampen activation
FcγRIIB inhibits signaling by recruiting SHIP phosphatase to counteract PI3K signaling
CD40 provides co-stimulatory signals enhancing B cell survival, proliferation, and isotype switching upon binding CD40L
Toll-like receptors (TLRs) recognize pathogen-associated molecular patterns synergizing with BCR signaling to enhance B cell activation