Complement-mediated inhibition of immune precipitation. II. Analysis by sucrose density gradient ultracentrifugation.

The factors influencing the ultracentrifugation characteristics of immune complexes generated in the presence of fresh normal human serum have been analysed. In the absence of alternative pathway factors B, D or Properdin, the size of complexes was increased. When classical pathway function was blocked, in C1q deficient serum or in the presence of Mg EGTA, although the proportion of complexes remaining in solution were reduced their size was similar to those formed in normal human serum. In C2 deficient serum, a heterogeneous population of complexes was generated. In all instances repletion with the appropriate missing complement component reversed the abnormality. We conclude that there is normally a rapid sequential process of classical followed by alternative pathway activation leading to stable soluble complexes. In the absence of C1 activation the alternative pathway process requires precipitation of the antigen-antibody aggregates whereas in normal serum these events occur in the fluid phase. We suggest that in C2 deficient serum the C1 and/or C4 reacted complexes fail to activate the alternative pathway efficiently.