Complement and meningococcal infection.

Serum C3 levels were measured in 211 patients with meningococcal disease. Low levels were found in 13 patients with acute meningococcaemia, and complement activation may have contributed to the peripheral circulatory collapse that was responsible for nine deaths. The complement profile of these patients suggested activation of both classical and alternative complement pathways. Patients with meningitis had a higher mean serum C3 level than controls. Serial studies in 13 serum antigen-positive patients with meningitis who subsequently developed arthritis or cutaneous vasculitis showed a transient fall in serum C3 in eight. This fall was probably due to the formation of immune complexes that were responsible for their allergic complications.