Quantitation of the human component C4: definition of C4 Q0 alleles and C4A duplications.

We determined the C4 plasma concentrations of 48 genotypically CA4- and C4B-defined unrelated individuals from 34 families with a total of 196 members by an enzyme-linked immunosorbent assay using Rg:1,2 (C4A) and Ch:1 (C4B) specific monoclonal antibodies. The results obtained allowed the establishment of rules for the detection of C4 Q0 alleles in the heterozygous form and of C4A gene duplications. In the present study seven homoduplications of the C4A 3 allotype were defined which had not been detected by allotyping. This procedure allows the simple, reliable, and quick determination of Rg:1,2 and Ch:1 plasma levels which are not influenced by daily rhythms of C4 production.