PRIMARY immunodeficiency diseases are a heterogeneous group of disorders resulting from intrinsic defects of the immune system.1 They are frequently associated with repeated bacterial, fungal, or viral infections and may be caused by defects in organs (thymus), cells (B lymphocytes and phagocytes), or molecules (immunoglobulins, complement, adhesion receptors, and enzymes) of the immune system. Primary immunodeficiencies involving T lymphocytes or T-cell–specific molecules have only recently begun to be recognized, and thus children with recurrent infections due to faulty T-lymphocyte function may be given an incorrect diagnosis because of the lack of suitable laboratory tests.2 Understanding of T-cell physiology has lagged… © 1992, Massachusetts Medical Society. All rights reserved.