Basal serum immunoglobulin levels in wild-type and B7-H2^–/– mice. The serum immunoglobulin levels of various isotypes for each individual wild-type (•) and mutant (○) mouse were measured via ELISA using known standards and plotted. Statistical signifi-cance was established using a paired 2-tailed Student t test, and the P values are indicated. Horizontal bars indicate the mean.