Accordingly, CD154 deficiency is associated with an impairment of the humoral immune response to T-cell dependent antigens, including defective immunoglobulin class switching; patients with the X-linked hyper-IgM syndrome caused by mutations of the CD154 gene, generally present low serum IgG and IgA, but normal or increased serum IgM, and are susceptible to opportunistic infections. This evidence concerns the gene CD40LG and X-linked hyper-IgM syndrome.