Passive immunization of FcR and complement deficient mice. C57Bl/6, FcR k/o, C2/fB double k/o (completely complement deficient), Classical (possessing only the classical complement pathway), Lectin (possessing only the lectin complement pathway), and Alternative (possessing only the alternative complement pathway) mice were injected i.p. with 300 μl of either naïve or immune mouse serum 1 day prior to i.p. challenge with 105 C. burnetii. Severity of infection was determined at 2 weeks post-challenge by assessing spleen weights (A) and bacterial genome equivalents per spleen (B).