Radioimmunoassay for anaphylatoxins: a sensitive method for determining complement activation products in biological fluids.

Activation of the blood complement system generates bioactive fragments called anaphylatoxins. The three anaphylatoxins C3a, C4a, and C5a are released during "classical pathway" activation while only C3a and C5a are released when the "alternative pathway" of complement is activated. Radioimmunoassays were designed to individually detect and quantitate the activation fragments C3a, C4a, and C5a in biological fluids without interference from the precursor molecules C3, C4, and C5. Kinetics of complement activation in fresh human serum exposed to the activators zymosan, heat-aggregated immunoglobulin, or cobra venom factor were monitored using the radioimmunoassay technique. For the first time, activation of components C3, C4, and C5 was followed simultaneously in a single serum sample. Analysis of the patterns and extent of anaphylatoxin formation during activation in serum may be used to screen for deficiencies or defects in the complement cascade. Levels of the anaphylatoxins in freshly drawn serum were much higher than levels detected in EDTA-plasma. Detection limits of anaphylatoxins in plasma are governed by background levels of 152 +/- 69, 155 +/- 33, and 5.4 +/- 6.6 ng/ml for C3a, C4a, and C5a, respectively. Detection of low-level complement activation in patient's blood, urine, or synovial fluid, using anaphylatoxin formation as an indicator, may prove useful in signaling numerous forms of inflammatory reactions. The demonstration of anaphylatoxins in clinical samples is being recognized as a valuable diagnostic tool in monitoring the onset of immune disease.