Radioassays for quantitation of intact complement proteins C2 and B in human serum

Availability of polyclonal and monoclonal antibodies recognizing determinants on the major cleavage fragments of complement proteins C2 and B enabled development of sensitive radioassays which can be used to quantitate the intact proteins in human sera. Changes in C2 and B concentrations indicative of classical or alternative pathway activation, or both, were seen in normal serum after incubation with complement activators. We determined the normal range (x ± 2 SD) of C2 concentration to be 11–35 μg/ml in 32 healthy individuals, and that of protein B to be 74–286 μg/ml. Sera from patients with systemic lupus erythematosus (SLE), septic shock, infections, and following orthopedic surgery were then assayed. Mean protein B concentration was significantly higher in SLE sera (P = 0.002) and in the infected and post-operative (acute-phase) sera (P < 0.001), and the mean C2 concentration in the septic shock group (P < 0.001) was significantly lower than the mean of healthy individuals. Intact C2 was not detected in known C2-dependent individuals. These assays allow parallel quantitation of the structurally and functionally homologous proteins of the classical (C2) and alternative (B) pathways, which is of interest in patients with genetic and acquired hypocomplementemia.