Antibodies of the adaptive immune system can bind antigen, forming an antigen-antibody complex.

The 80-amino acid helical component of each triple peptide contain many Gly-X-Y sequences, where X and Y are proline, isoleucine, or hydroxylysine; they, therefore, strongly resemble collagen fibrils.

[2] The C1q protein is produced in collagen-producing cells and shows sequence and structural similarity to collagens VIII and X.

Genetic deficiency of C1q is extremely rare (approximately 75 known cases) although the majority (>90%) of those have SLE.

[5] C1q associates with C1r and C1s in order to yield the C1 complex (C1qr2s2), the first component of the serum complement system.