The 7E9 antibody specifically recognizes CD21/CD35, an epitope shared by mouse Complement receptor 2 (CR2) and Complement receptor 1 (CR1), which are also known as CD21 and CD35, respectively. Mouse CD21 and CD35 are ~145-kDa and ~190-kDa single pass type I transmembrane glycoproteins, respectively. Unlike human CD21 and CD35 that are encoded by separate CR2 and CR1 genes, mouse CD21 and CD35 are encoded by a single Cr2 gene and are generated by alternative mRNA splicing. CD21 and CD35 are expressed on B cells and follicular dendritic cells (FCD) but not on thymocytes, T cells, erythrocytes, or platelets. CD35 is also expressed on macrophages and activated granulocytes. CD21 binds to cleaved C3b fragments, including C3d- and C3dg-bound to antigen, whereas CD35 binds to C3b-, iC3b-, and C4b-bound complexes. These receptors play many important roles including the transport, presentation, and retention of complement fragment-tagged antigens and immune complexes crucial for generating strong B cell antibody-producing responses. Complement-tagged antigen can bind to CD21, part of the B cell coreceptor complex and to the B cell receptor for antigen (BCR) which facilitates the antigen-driven activation of B cell responses. CD35 interacts with other regulatory factors to promote C3b and C4b degradation as well as playing roles in stimulating phagocytic functions in activated leucocytes. The 7E9 antibody recognizes an epitope on CD35 distinct from the epitope recognized by the mouse CD35-specific clone, 8C12, as well as the mouse CD21/CD35-specific clone, 7G6.