The CD19-Coupled CD21 is an Essential Receptor for B cell Activation

It has been almost six decades since T cells and B cells were found to be the major players of adaptive immunity. The signal transduction cascades of T cell activation are well studied. However, it is not well appreciated or investigated how the binding of antigens to B cell receptors triggers the downstream signal transduction pathways. It is clear that MHCs with linear antigens presented by antigen-presenting cells (APCs), binding to TCR coupled with CD4/8, is enough to trigger the activation of T cells. For B cells, there are structures of BCRs similar to those of TCRs; however, there are no definitive structures similar to CD4/8 in B cells to help B cell activation. It remains a mystery how B cells are activated by native forms of antigens. Encouraged by our new illustration of the underlying mechanism of T cell activation, a simple model of B cell activation is derived: the CD19-coupled CD21 is an essential receptor for B cell activation.