CD21 (CR2) and CD35 (CR1) are key complement receptors predominantly expressed on B cells and follicular dendritic cells (FDCs). CD21, also known as the C3d receptor, binds to complement fragments like C3d and iC3b, playing a crucial role in B cell activation and regulation of immune responses. It also serves as the receptor for Epstein-Barr virus (EBV) on B cells. CD35, on the other hand, binds to C3b and C4b, facilitating phagocytosis and immune complex clearance. It helps in the activation of B cells and regulation of complement activity. Both receptors are involved in antigen presentation and immune complex internalization, contributing to both innate and adaptive immune responses.
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