The primary function of CR1 is to serve as the cellular receptor for C3b and C4b, the most important components of the complement system leading to clearance of foreign macromolecules. The Knops blood group system is a system of antigens located on this protein. Follicular dendritic cells (FDC) are restricted to the B-cell regions of secondary lymphoid follicles. They are CD21+/CD35+/CD1a-.