CD40LG and severe combined immunodeficiency: Thus, a lack of B cells in X-linked agammaglobulinemia (XLA) a lack of T and NK cells in X-linked several-combined immunodeficiency (X-SCID) and impaired B-cell responses in X-linked hyper-IgM syndrome due to mutations in BTK, IL2RG, and CD40LG, respectively, predispose affected individuals to severe, recurrent, and often life-threatening infections [10],[55].