The C1q complex is potentially multivalent for attachment to the complement fixation sites of immunoglobulin. The sites are on the CH2 domain of IgG and probably on the CH4 domain of IgM. The appropriate peptide sequence of the complement fixing site might become exposed following complexing of the immunoglobulin, or the sites might always be available, but might require multiple attachment by C1q with critical geometry in order to achieve the necessary avidity. C1q is a 400K(K=1000) 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. 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. It is assumed that the globular ends are the sites for multivalent attachment to the complement fixing sites in immune complexed immunoglobulin Schematic of antibody binding to an antigen An antibody is a protein complex used by the immune system to identify and neutralize foreign objects like bacteria and viruses. ... Peptide sequence or amino acid sequence is the order in which amino acid residues, connected by peptide bonds, lie in the chain. ... Avidity is a measure of the synergism of the strength individual interactions when they work as a group. ... L-Proline is one of the twenty proteinogenic units which are used in living organisms as the building blocks of proteins. ... Isoleucine is one of the 20 natural amino acids, and is coded for in DNA. Its chemical composition is identical to that of leucine, but the arrangement of its atoms is slightly different, resulting in different properties. ... Hydroxylysine is an amino acid, C6H14N2O3. ... Tropocollagen triple helix. ... fine fibers (e. ...