Given the short time between inoculation and symptoms of arthritis in our model, our in vivo findings and the reduced in vitro phagocytosis of bacteria incubated with C3-deficient but not fB-deficient serum could be at least partly due to lower levels of specific antibodies against S. aureus present a priori in the blood of mice and not the specific antibody response triggered by the inoculation as such. Here, CFB is linked to Arthritis.