C1q is composed of 18 polypeptide chains: six A-chains, six B-chains and six C-chains. Each chain contains a collagen-like region located near the N terminus and a C-terminal globular region. Circulating C1q forms together with the proenzymes C1r and C1s the C1 macromolecule the first component of the classical complement pathway.
The formation of an antibody–antigen complex (immune complex) is the principal way of activating the classical pathway of the complement system. With the Fc regions of IgG and IgM antibodies present in immune complexes, efficient activation of the classical pathway is initiated. In this way, C1q acts to bridge the innate and adaptive immune systems.
Furthermore, C1q can bind to early apoptotic cells, where it activates the classical complement pathway and mediates phagocytosis. As such, C1q promotes the clearance of apoptotic cells and subsequent exposure of auto-antigens, thereby preventing stimulation of the immune system. C1q deficiency has a profound effect on host defense and clearance of immune complexes and is particularly associated with lupus erythematosus and glomerulonephritis.
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FC = Flow Cytometry, Intracellular Staining/Flow Cytometry; ELISA = ELISA, ELISPOT; Multiplex = Multiplex Immunoassays; ICC = Immunocytochemistry; IHC = Immunohistochemistry, Immunofluorescence, Microscopy, Imaging, In Vivo Imaging; IHC-F = Immunohistochemical Staining of Frozen Tissue Sections; IHC-P = Immunohistochemical Staining of Formalin-Fixed Paraffin Embedded Tissue Sections; FA = Functional Assays, Bioassays, Neutralization, Depletion Studies, Biomolecule Conjugation; IP = Immunoprecipitation; WB = Western Blotting
RUO = Research Use Only; GPR = General Purpose Reagent; ASR = Analyte Specific Reagent. Analytical and performance characteristics are not established; CE = CE-marked reagents