When encapsulated type 25 pneumococci (Pn25) were opsonized with normal guinea pig serum, they consumed much more C3 than other complement (C) components. Fixation of C3 to the organisms was demonstrated by radio-labeling techniques, and its capsular localization was established by the use of monospecific anti-C3 antibody.

Treatment of the serum with an appropriate dose of a purified cobra venom factor (VF) destroyed C3 and all of the opsonic activity, without appreciably affecting the other C components. Addition of purified C3 completely restored the opsonic activity of the VF-treated serum, indicating a requirement for C3. Since purified C3 alone had no opsonic activity, it was concluded that the C3 molecules had to be cleaved (to C3b) to function as opsonins.

Experiments with C5-deficient mice revealed that C5 also plays a definite, but quantitatively less impressive, role in antipneumococcal defense.

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