IgG2 inhibits serum-mediated killing. (A) Titers of IgG1 and IgG2 isotypes in patient serum and HCS that is specific to infecting strains. Dashed line indicates the median IgG2 titer from sera S4–S11. Data are representative of three independent experiments. ***, P < 0.001. (B) Killing of B1 was measured with S1 depleted of IgG2 mixed with HCS or with a mixture HCS supplemented with IgG2 purified from S1 or HCS alone. For all data, error bars represent the mean ± SD for a minimum of three independent experiments.