The quantitation of C6 in rabbit and human sera.

C6 quantitation was carried out in rabbit and human sera by the single radial immunodiffusion technique. The C6 content of the rabbit and human sera used as standards was estimated by precipitin analysis, using an anti-C6 antiserum labelled with I. The mean C6 level in normal human serum was 11 μg/ml, whereas in normal rabbit serum it was 35 μg/ml. Sera from forty rabbits heterozygous for C6 deficiency were found to have a mean concentration of C6 of 14 μg/ml. The C6 level was estimated in sera from patients with various immunological disorders and found to be significantly higher in the sera from patients with rheumatoid arthritis and normal in the sera from patients with SLE, glomerulonephritis, nephrotic syndrome and myeloma. C6 haemolytic assays were found to correlate well with the antigenic assays only in fresh sera. In various circumstances this correlation breaks down, presumably because of C6 inactivator. This inactivator, in contrast to C3-inactivator, appears to be bound to antigen–antibody complement complexes.