AbstractStreptococcus pneumoniae is an important human pathogen and the most common cause of community-acquired pneumonia. Both adaptive and innate immune mechanisms provide protection from infection. Innate immunity to S. pneumoniae in mice is mediated by naturally occurring anti-phosphocholine (PC) Abs and complement. The human acute-phase reactant C-reactive protein (CRP) also protects mice from lethal S. pneumoniae infection. CRP and anti-PC Ab share the ability to bind to PC on the cell wall C-polysaccharide of S. pneumoniae and to activate complement. CRP and IgG anti-PC also bind to FcγR. In this study, FcγR- and complement-deficient mice were used to compare the mechanisms of protection conferred by CRP and anti-PC Ab. Injection of CRP protected wild-type, FcRγ-chain-, FcγRIIb-, and FcγRIII-deficient mice from infection. Complement was required for the protective effect of CRP as cobra venom factor treatment eliminated the effect of CRP in both γ-chain-deficient and wild-type mice, and CRP failed to protect C3- or C4-deficient mice from infection. Unexpectedly, γ-chain-deficient mice were extremely sensitive to pneumococcal infection. This sensitivity was associated with low levels of natural anti-PC Ab. γ-chain-deficient mice immunized with nonencapsulated S. pneumoniae produced both IgM- and IgG PC-specific Abs, were protected from infection, and were able to clear the bacteria from the bloodstream. The protection provided by immunization was eliminated by complement depletion. The results show that in this model of systemic infection with highly virulent S. pneumoniae, protection from lethality by CRP and anti-PC Abs requires complement, but not FcγR.