CD40 Ligand Deficiency

**Semantic type:** Disease or Syndrome

**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.