Serum Ig levels in p38α^−/− chimeric mice. Serum levels of IgM, IgA, IgG3, IgG1, and IgG2b were measured by ELISA from three p38α^+/+, five p38α^+/−, and seven p38α^−/− chimeric mice that were 6–8 wk old, as described in Materials and Methods.