The C1q receptor site on human immunoglobulin G.

Of the many functional properties of the immunoglobulin G (IgG) molecule, only antigen binding and the interaction with the C1q component of complement have been shown to be uniquely associated with the individual compact domains which make up this immunoglobulin. The chemical and biological evidence for the exclusive association of the C1q-binding site with the CH2 domain is reviewed, affirming that the site probably is centered on the residues 279-295 which are located on the outside of the molecule and contain, in particular, three positively charged residues which are thought to be vital to the interaction. An alternative site (residues 316-338), having similar exposure and charge characteristics, is discussed and arguments are presented indicating why the former is currently favoured.