A young patient with severe recurrent pyogenic bacterial infections, but with normal T-cell and B-cell numbers, arrives at the hospital. Testing reveals that this patient's CD4 T-helper cells have a defect in CD40 ligand. As a result, humoral immunity evaluation reveals a significant elevation in the levels of which immunoglobulin that is present as a monomer on B-cell surfaces, as a pentamer in serum, and is initially seen in the primary immune response?

IgM antibody, in contrast, arises early in infection and then disappears within a couple of months. IgM is intravascular and does not cross the placental barrier. For this reason, infants with specific IgG responses to disease must be tested for IgM to determine whether their immune systems have produced antibody or whether the test was positive because of passively transferred IgG from mother to child. The clinical case is of a young patient with hyper-IgM syndrome. This immune deficiency presents early in life with severe pyogenic infections, resembling those seen in X-linked hypogammaglobulinemia; however, unlike X-linked hypogammaglobulinemia (very low levels of all immunoglobulins, virtual absence of B cells, found in young boys with female carriers being immunologically normal), hyper-IgM syndrome patients have a high IgM concentration, low IgG, IgA, and IgE concentrations, and normal T- and B-cell numbers. The problem exists in a mutational defect in the CD40 ligand in CD4-positive helper T cells, leading to failure of normal interaction between this ligand on T cells and CD40 on B-cell surfaces. This failure leads to an inability of the B cells to class switch from IgM production to other antibodies. Treatment includes pooled gamma globulin.