The
B-cell co-receptor complex
• Similar to T-cell
signaling, B-cell signaling is also
enhanced by aggregation of the BCR with a co-receptor.
• One way of
co-ligation (cross-linking of the BCR
and co-receptor) is through CD21 binding of complement component, C3d, already bound to antigens directly or through the action of bound antibodies.
• CD19 then becomes
tyr phosphorylated by BCR
associated kinases, leading to the binding of Src kinases to the cytoplasmic domain of CD19 and the recruitment of phosphatidylinositol 3-OH kinase (PI 3-kinase). This enhances the transduced signal 1000 to 10,000 fold.
•CD19 is expressed before CD21 and contributes to BCR and pre-BCR signaling even when the co-receptor has not bound complement through CD21