Studies into the occurrence of soluble antigen--antibody complexes in disease. I. A biological assay for soluble complexes.

Soluble antigen–antibody complexes (SC) have in the past been shown to activate histamine release from the isolated perfused guinea-pig lung. In the current studies this system has been used as a biological assay for the detection of SC present in serum. However, serum contained an inhibitor substance which suppressed the histamine-releasing activity of SC. The presence of this inhibitor resulted in the bioassay being relatively insensitive, with a capacity to detect SC present in serum at a minimum concentration of approximately 32–128 μg antibody N/ml. By fractionating the serum on Sephadex G-200, it was possible to separate the inhibitor (middle fraction) from the SC (excluded fraction), resulting in a capacity to detect SC which were initially present in serum at concentrations of approximately 3·2–8 γg antibody N /ml. There was a more efficient recovery of complexes in the Sephadex excluded fraction obtained from SC containing intermediate and late response antibody than from SC containing early response antibody. Most of the free antigen present in the starting preparations of SC became separated from the complexes during passage through Sephadex G-200; however, the amount of antigen recovered with antibody in the Sephadex excluded fraction was in excess of that required for equivalence. It was concluded that the isolated perfused guinea-pig lung could be used as a sensitive bioassay for the detection of SC present in the Sephadex excluded fraction of serum.