Immunodeficiency Disorders

The immunologic disorders discussed up to now can be viewed as resulting from the overreaction of the body's immune systems. By contrast, in immunodeficiencies, the body is incapable of making or sustaining an adequate immune response. There are two basic types of immunodeficiency diseases: primary, or congenital; and secondary, or acquired. Primary immunodeficiency can be inborn as the result of a genetic defect or can result from developmental abnormalities. Secondary immunodeficiency can be acquired as the result of infection or other stresses on the immune system such as malnutrition. People with either type of immunodeficiency are subject to repeated infections. The types of these infections will often depend on which part of the immune system is absent or malfunctioning. Some of the more important immunodeficiency diseases are listed in table 18.5.

Primary Immunodeficiencies

The genetic or developmental abnormalities that cause primary immunodeficiencies affect B cells, T cells, or both. Some primary immunodeficiencies affect natural killer (NK) cells, phagocytes, or complement components. Primary immunodeficiencies are generally rare. Agammaglobulinemia, a disease in which few or no antibodies are produced, occurs in one in about 50,000 people, and severe combined immunodeficiency (SCID), where neither T nor B lymphocytes are functional, occurs in only about one of 500,000 live births.

Selective IgA deficiency, in which very little or no IgA is produced, is an exception in that it is not rare. It is the most common primary immunodeficiency known. It has been reported in different studies to occur as often as one per 333 to 700 people. Although people with this disorder may appear healthy, many have repeated bacterial infections of the respiratory, gastrointestinal, and genitourinary tracts, where secretory IgA is normally protective.

Primary deficiencies may occur in various components of the complement system. For example, the few individuals who lack C3 are prone to develop severe, life-threatening infections with encapsulated and pyogenic bacteria. Patients with deficiencies in the early components of complement such as C1 and C2, may develop immune complex diseases, because these components normally help to clear immune complexes from the circulation. Patients who lack late components of the classical pathway of C activation (C5, C6, C7, C8) have recurrent Neisseria infections. Immunity to these bacteria is associated with destruction of the organisms by complement-dependent bactericidal antibodies. The effects of defects in other parts of the complement system are reflected by various symptoms, depending on the deficient component, but in general there is an increase in infections. People who lack one of the important control proteins of the sequence, C1-inhibitor, experience uncontrolled complement activation. This causes fluid accumulation and potentially fatal tissue swelling, a condition called hereditary angioneurotic edema. ■ complement system, p. 381

In children with DiGeorge syndrome, the thymus fails to develop in the embryo. As a result, T cells do not differentiate and are absent. Affected individuals have other developmental defects as well, such as heart and blood vessel abnormalities, and a characteristic appearance with low-set deformed ears, small mouth, and wide-set eyes. As expected from a lack of T cells, affected people are very susceptible to infections by

1 Nester-Anderson-Roberts:

III. Microorganisms and 1 18. Immunologic Disorders 1

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