CD40 and Opportunistic infection: Since the CD40/CD40L pathway also mediates co-stimulation between activated CD4+ T cells and CD40-expressing monocytes, macrophages, and dendritic cells, mutations in either CD40L (X-linked) or in CD40 (AR) also lead to opportunistic infections, notably Pjp [257,298,299,300,301,302,303,304,305,306,307,308,309,310].