CD40LG and agammaglobulinemia: Most APDS patients have elevated transitional B cells, reduced class-switched memory B cells, variable immunoglobulin levels (mainly reduced IgG and increased IgM levels, hypogammaglobulinemia, or in some cases agammaglobulinemia) associated with a poor vaccine response, and an impaired in vitro B cell isotype switching (47, 51, 64, 69).