C1q is a 400 KDa protein formed from 18 peptide chains in 3 subunits of 6. Each 6 peptide subunit consists of a Y-shaped pair of triple peptide helices joined at the stem and ending in a globular non-helical head.C1q is alarge molecule made up of 18 polypeptide chain s(6 of A,6 of B,and 6 of C)held together by acombination of covalent(disulfide) and noncovalent bonds.

When assembled,the C1q full molecule has two distinct regions:a globular head and a longtail.Most of the antibodies raised against C1q specific to the secondary structure of the assembled chains and are nonreactive to the amino acid sequence of each chain.The tightly bound individual polypeptide chains are difficult to separate from this complex,which requires long incubations under reducing conditions and high urea concentrations.Several proteins have globular domains similar to C1q.