Immune Disorders Involving The Thymus

Several immunodeficiency syndromes are grounded in failure of the thymus to undergo normal development. Thymic malfunction has a profound effect on T-cell function; all populations of T cells, including helper, cytolytic, and regulatory varieties, are affected. Immunity to viruses and fungi is especially compromised in those suffering from these conditions.

DiGeorge syndrome, or congenital thymic aplasia, in its most severe form is the complete absence of a thymus. This developmental defect, which is associated with the deletion in the embryo of a region on chromosome 22, causes immunodeficiency along with characteristic facial abnormalities, hypoparathyroidism, and congenital heart disease (Figure 19-4). The stage at which the causative developmental defect occurs has been determined, and the syndrome is sometimes called the third and fourth pharyngeal pouch syndrome to reflect its precise embryonic origin. The immune defect includes a profound depression of T-cell numbers and absence of T-cell responses. Although B cells are present in normal numbers, affected individuals do not produce antibody in response to immunization with specific antigens. Thymic transplantation is of some value for correcting the T-cell defects, but many DiGeorge patients have such severe heart disease that their chances for long-term survival are poor, even if the immune defects are corrected.

Whereas the DiGeorge syndrome results from an in-trauterine or developmental anomaly, thymic hypoplasia, or the Nezelof syndrome, is an inherited disorder. The mode of inheritance for this rare disease is not known and its presentation varies, making it somewhat difficult to diagnose. As the name implies, thymic hypoplasia is a defect in which a vestigial thymus is unable to serve its function in T-cell development. In some patients, B cells are normal, whereas in others a B-cell deficiency is secondary to the T-cell defect. Affected individuals suffer from chronic diarrhea, viral and fungal infections, and a general failure to thrive.

Digeorge Ears

FIGURE 19-4

A child with DiGeorge syndrome showing characteristic dysplasia of ears and mouth and abnormally long distance between the eyes. [R. Kretschmereta!., 1968, New Engl. J. Med. 279:1295; photograph courtesy of F. S. Rosen.]

FIGURE 19-4

A child with DiGeorge syndrome showing characteristic dysplasia of ears and mouth and abnormally long distance between the eyes. [R. Kretschmereta!., 1968, New Engl. J. Med. 279:1295; photograph courtesy of F. S. Rosen.]

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