cross-linking of immunoglobulin by antigen is essential but not always sufficent to initiate the signal cascade for Bcell activation. stimulation of additional receptors is also necessary for the full activation and differientiation of naive B cells. Describe these receptors and their ligands & outline how they help activate the B cell.
B-cell coreceptor made of CD21, CD19, and CD81 cooperate with Bcell receptor and B cell activation and increases the sensitivity of the B cell to antigen 1,000-10,000 fold. This becomes important when concentrations are low. CD21 binds to complement components that have been deposited on the surface of pathogens. CD19 provides the long cytoplasmic tail involved in signaling. When the co-receptor and Bcell receptor and co-receptor are both ligated by antigen and C3d. Lyn is a protein tyrosine kinase bound to the ITAMs of Iga which phosphorylates CD19 when it is nearby. The phosphorylated CD19 tail will initiate activation signals that complement those generated by the Bcell receptor complex.
Depending on the characteristics of the antigen an additional signal provided by CD4 Th2 cells may be needed for Bcell activation. Th2 cells express CD40 ligand on their surface which binds to CD40 on B cell and delivers a stimulatory signal. They also secrete cytokines IL-4, IL-5, and IL-6. By binding to specific receptors these help stimulate B cells to proliferate and differientiate into plasma cells