Complement C1q

In medicine and immunology, complement C1qis a "subcomponent of complement C1, composed of six copies of three polypeptide chains (A, B, and C), each encoded by a separate gene (C1QA; C1QB; C1QC). This complex is arranged in nine subunits (six disulfide-linked dimers of A and B, and three disulfide-linked homodimers of C). C1q has binding sites for antibodies (the heavy chain of immunoglobulin G or immunoglobulin M). The interaction of C1q and immunoglobulin activates the two proenzymes complement C1r and complement C1s, thus initiating the cascade of complement activation via the classical complement pathway".

The C1q level is low in acquired C1 esterase inhibitor deficiency (acquired angioedema).