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.