The immune system contains multiple components that provide protection against specific groups of microorganisms. The degree of specialization of function is clearly illustrated in patients with immunodeficiency syndromes in whom the nature of the defect determines the sites of the infections and the susceptibility to certain organisms. Chronic mucocutaneous candidiasis is an especially dramatic illustration of the role of the T-lymphocyte system in defense against opportunistic fungal infections, especially of the skin and mucous membranes. There is considerable heterogeneity of the immune abnormalities in patients with chronic mucocutaneous candidiasis. The most consistent defects involve subnormal production of lymphokines by T-cells in response to Candida antigens. However, some patients have more global defects and are unresponsive to all antigens and may have reduced responsiveness to mitogens as well. Successful therapy of this infection requires a combination of treatments, including such antifungal drugs as clotrimazole, ketoconazole, or amphotericin B and correction of the underlying immune defect with such agents as transfer factor.