Recognizes a protein of 210-220kDa, which is identified as the complement receptor 1 (CR1)/CD35. This MAb does not block CR1 activity. It is highly specific to CR1 and shows no cross-reaction with CR2. 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-. This MAb labels follicular dendritic cells and follicular dendritic cell sarcoma.