Significance of sequential changes in serum complement levels during acute anaphylactoid reactions.

Reduced levels of complement fractions C3 and C4 and total hemolytic complement (CH50) are usually attributed to classical complement pathway activation. However, studies in 26 patients undergoing severe anaphylactoid reactions during general anesthesia suggest that these changes may equally reflect plasma dilution and protein redistribution. Twelve patients showed a dramatic fall in complement complexes which tended to resolve after 24 h; however, this fall was not usually associated with the detection of C3 breakdown products or antigen-antibody complexes. Measurement of complement levels is not a reliable index of activation or cleavage when there are dynamic shifts in plasma volume.