B cell activated co-receptor plays important roles in linkage of innate and acquired humoral immune responses. CD21 molecule in the co-receptor complex is a receptor for C3dg and CD19 molecule enhances BCR signal transduction. CD21 also expresses on the surface of follicular dendritic cells, which mediates the long-term maintenance of antigens and is indispensable for maintaining the memory of B cells. B cell activated co-receptor also has an effect on the negative selection of B cells reactive to autoantigens.