The characteristic immunophenotype includes variable serum Ig levels (ranging from hypogammaglobulinemia to hypergammaglobulimemia to those with a hyper IgM phenotype), reduced Ag-specific antibody titers, peripheral T and B cell subset lymphopenia with inverted CD4/CD8 ratios, low numbers of isotype-switched memory B cells, and evidence of impaired cytotoxic potential of CD8+ T cells and NK cells [64]. This evidence concerns the gene CD8A and agammaglobulinemia.