These observations are analogous to mutant mice deficient in secreted IgM, in that immunoglobulin isotype switching to IgG is normal; however, these animals are also unable to control viral, bacterial, and fungal infections due to impaired induction of a protective specific IgG antibody response, and lack of serum IgM (4, 27, 28). Here, CD40LG is linked to fungal infectious disease.