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Immunodeficiency can occur in any of the four major components of the immune system: (1) B cells (antibody), (2) T cells, (3) complement, and (4) phagocytes. In a patient with a history of infections that are unusually frequent, unusually severe, or caused by unusual organisms, the pattern of these infections can indicate which component(s) of the immune system might be defective. Most immunodeficiencies are acquired, and these are frequently caused by immunosuppressive medications, transplantation, and/or diseases that suppress immunity. Although they are less common, congenital immunodeficiencies (Table 68–1) are important to understand because (1) the patterns of infections that are seen teach us how various immune components are supposed to function normally, and (2) recent technological advances have allowed us to better diagnose and treat these diseases, preventing the infectious complications.
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CONGENITAL IMMUNODEFICIENCIES
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Congenital T-cell deficiencies tend to be the most severe and easily recognized immunodeficiency. Because T cells are central to so many aspects of immune responses, including antiviral immunity, maturation of B cells and antibody, and the activation of macrophages, their absence results in a broad range of unusual opportunistic infections (i.e., viruses, bacteria, fungi, and protozoa that are rarely seen in healthy hosts). These diseases often present within the first 6 to 12 months of life ...