Classical, lectin, and alternative complement pathway activation byN meningitidisin plasma from the patient and his mother. The formation of C1rs-C1inh complexes (marker of classical pathway activation), C4bc (marker of classical or lectin pathway), and C3bBbP (marker of the alternative pathway) in plasma of the homozygous factor D–deficient patient (□), his heterozygous mother (▪), and the median (± IQR) of 4 healthy controls (▦). The plasma was incubated for 1 hour at 37°C without stimulus or with 10⁸/mL meningococci, in the absence or presence of recombinant factor D or anti–factor D, as indicated. The experiment was repeated once, yielding virtually identical results (not shown).