Medical Genetics The immunologic defect in the x- linked form is a deficiency of T cell gp39, the ligand for CD40 on B cells that induces switching from IgM to IgA, IgG, and igE. The mutated gene has been identified at Xa27Medical Genetics The immunologic defect in the X￾linked form is a deficiency of T cell gp39, the ligand for CD40 on B cells that induces switching from IgM to IgA, IgG, and IgE. The mutated gene has been identified at Xq27