Cross-linking of Ig by antigen is essential but not always sufficient to initiate the signal cascade for B cell activation. Stimulation of additional receptors is also necessary for the full activation and differentiation of naive B cells. Describe these receptors and their ligands, and outline how they help activate the B cell.
The B-cell co-receptor, made up of CD21, CD19 and CD81 (TAPA-1) cooperates with the BCR in B cell activation and increases the sensitivity of the B cell to antigen 1000-10000x. This becomes important when antigen concentrations are low. CD21 binds C3d, when the coreceptor and BCR are both ligated to antigen and C3d, Lyn and the cytoplasmic tail of CD19 come close together, leading to phosphorylation of CD19, which initiates activation leading to activation of the B cell.