B hermsii–induced thrombocytopenia does not require specific immunity. (A) Two CB.17/SCID (•, ▪) or 2 C57BL6/SCID (○, □) mice were infected intraperitoneally with 10⁶B hermsii strain DAH. Bacteremia (left) was measured by visually counting the number of spirochetes in infected blood under dark-field microscopy, and platelet counts (right) were determined by flow cytometry (see “Materials and methods”). Broken line indicates the detection limit for spirochetemia. (B) Platelet counts of mice infused with serum of uninfected (○) and infected thrombocytopenic (•) mice. Shown are the means ± SDs of 5 mice.