CD40LG and Opportunistic infection: 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.