NCIt definition : Patients with the X-linked hyper IgM syndrome have a deficiency of a protein, CD40
ligand that is found on the surface of T-lymphocytes. As a consequence of the deficiency
of this protein, their T-lymphocytes are unable to instruct B-lymphocytes to switch
their production of gamma globulins from IgM to IgG and IgA. As a result, patients
have decreased levels of IgG and IgA and normal or elevated levels of IgM. In addition,
since CD40 ligand is important to other functions of T-lymphocytes, they also have
a defect in some of the protective functions of their T-lymphocytes.;