Monoclonal antibody 7E9 recognizes mouse complement receptor type 1 (CR1) and mouse complement receptor type 2 (CR2). CR1 and CR2 are cell surface glycoproteins capable of binding activated fragments of the third and/or fourth complement components (C3 and/or C4). They play a role in clearing immune complexes, phagocytosis, complement regulation and immune regulation. Mouse CR1 (MCR1, 190 kD) is found on the surface of B lymphocytes, follicular dendritic cells, and has low levels on peritoneal macrophages and activated granulocytes. MRC1 has binding activity to C3b and acts as cofactor I-mediated C3b cleavage. Mouse CR2 (MCR2, 150 kD) is a type I transmembrane glycoprotein that binds to complement fragments (C3d(g), iC3b) and interferon (IFN)-α. MCR2 may be expressed on follicular dendritic cells on B lymphocytes. On human B lymphocytes, it acts as a receptor for Epstein-Barr virus (EBV). MCR2 mediates the formation of rosettes between B lymphocytes and Crd with E. MCR1 and MRC2 are closely related. They are all products of a single gene Cr2 formed by alternative splicing of mRNA. MCR2 corresponds to the carboxy terminal part of MCR1. This is the opposite of human CR1 (CD35) and CR2. The monoclonal antibody 7E9 does not inhibit the rosette formation between 2PK3 cells and Crd-coated SRBC indicator cells.