X-linked hyper-IgM syndrome (XHIM) is a rare congenital immunodeficiency disorder, characterised by a defect in both humoral and cellular immune responses. In XHIM, the membrane glycoprotein CD40 ligand (expressed on activated T-cells) is deficient, which compromises T-cell interactions with antigen-presenting cells. Patients with XHIM present with severe, recurrent infections, predominantly of the respiratory and gastrointestinal tract. The gene encoding the CD40 ligand is located on the long arm of the X chromosome.