In order to elucidate how much C3 is required to achieve full protection against C.ps., the course of induced pneumonia in terms of body weight and clinical score was not only investigated in homozygous C3+/+ (WT) and C3−/−, but also in heterozygous C3+/− mice (Figures 1A–C). Here, C3 is linked to susceptibility to pneumonia measurement.