Defects in genes encoding molecules required for the above processes, which operate within germinal centers, result in a form of antibody deficiency with elevated (or normal) IgM levels but lacking IgG, IgA and IgE. These conditions are called hyper-IgM (HIGM) syndromes. A key requirement for germinal center formation and function is the interaction of CD40 (belonging to the TNF-receptor superfamily) found on the surface of B cells with an “activation induced,” CD40-ligand (CD40L) protein expressed on the surface of CD4 lymphocites. Mutations in the CD40L gene or the CD40 gene result in X-linked (relatively common) and autosomal recessive (rare) HIGM, respectively. Patients with the CD40L deficiency suffer from recurrent bacterial infections typical of antibody deficiency. However, because CD40L function is required for optimal T-cell immunity, they also suffer from opportunistic infections characteristic of T-cell deficiency. About a third of patients with CD40L deficiency develop Pneumocystis pneumonia. Infections with cryptosporiodosis, toxoplasmosis and nontuberculous mycobacteria also occur in this condition. These opportunistic infections can be explained on the basis that the interaction of CD40L on activated T cells with CD40 expressed on the surface of macrophages and dendritic cells, which in turn undergo maturation and activation, is required for the optimum expression of antimicrobial immunity.