The monoclonal antibody 7H8 recognizes mouse CIq. Clq, a member of the 'defense collagen' family, is the first subcomponent of the Cl complex of the classical pathway of complement activation. Several functions have been assigned to the pattern recognition molecule Clq, which include antibody-dependent and independent immune functions like triggering of rapid enhanced phagocytosis resulting in efficient containment of pathogens or clearance of cellular debris, apoptotic cells and immune complexes , and is considered to be mediated by Clq receptors present on the effector cell surface. There remains some uncertainty about the identities of the receptors that mediate Clq functions. Some of the previously described Clq receptor molecules, such as gClqR and cClqR, now appear to have less of a role in Clq functions than in functions unrelated to Clq. Experiments with gene targeted homozygous Clq-deficient mice have suggested a role for Clq in modulation of the humoral immune response, and also in protection against development of autoimmunity. The first component of complement Cl is a complex of three glycoproteins - Clq, Clr, and Cls. Cls and Clr interact to form a Cazf-dependent tetrameric proenzyme complex, C lr,-C 1 s2, which makes contacts with the Clq collagen domain. Binding of Clq to immune complexes (IgG or IgM) via the gClq domain, is considered to induce a conformational change in the collagen region of Clq, which leads to the autoactivation of Clr which, in turn, activates Cls. The activated Cl complex then cleaves components C4 and C2 in the classical complement cascade.