Bruton's (X-linked) agammaglobulinemia

In 1952, Bruton described an X-linked deficiency in immunoglobulin production. These patients suffered

from recurrent bacteria-related bronchitis, otitis, and skin infection. Infections usually began at about age 6 months, as circulating maternal antibodies in the infants subsided. All immunoglobulins were either markedly decreased or absent. Circulating mature B-cells were also absent.

It is now recognized that pre-B-cells are present in patients with Bruton's agammaglobulinemia, and there seems to be a defect in the B-cell maturation sequence. Morphologically, there are no germinal centers in lymph nodes, the spleen, and the tonsils. Plasma cells are absent.

Transient hypogammaglobulinemia of infancyis characterized by diminished levels of immunoglobulin but the ability to produce certain antibody. The disorder appears to be related to an abnormally long delay in the production of serum immunoglobulin. (Maternal antibodies normally decline in the infant over the first 6 months of life.) Defects in helper T-cell function are thought to be responsible.

Common variable immunodeficiencyis a somewhat poorly defined entity characterized by hypogammaglobulinemia despite normal numbers of circulating B-cells. In most cases, there is no clear-cut genetic predisposition. Patients range in age from very young to elderly, although young adults are most commonly affected.

Selective IgA deficiencyis the most common immunodeficiency disorder, occurring in about 1 in

700 people. Serum IgA levels are low, but the numbers of circulating IgA-cells are normal. However, these IgA-cells possess an immature phenotype that coexpresses IgD and IgM. Thus, the defect seems to be in the maturation of IgA-bearing cells. Similar selective deficiencies in IgM are reported but rare.

Combined syndromes—insufficiency of cellular and humoral immunity (T-, B-cell). This is inherited according to autosome-recessive type, e.g. Glanzmann-Riniker syndrome (agammaglobulinemia of Swiss type, Severe combined immunodeficiency (SCID). Hypoplasia of thymus and peripheral lymphatic tissue.

Severe combined immunodeficiency (SCID)is a severe disorder characterized by near total absence of both T-cell and B-cell immunity. Infants present early with recurrent opportunistic infections. SCID may be transmitted as either an X-linked or autosomal recessive trait.

Morphologically, there is a virtual absence of lymphoid tissue in the form of lymph nodes, spleen, and tonsils. The thymus gland fails to descend from the neck into the mediastinum and lacks lymphoid cells and Hassall's corpuscles.

These patients appear to have a stem-cell defect. A deficiency in the enyzme adenosine deaminase (ADA) is found in the cells of many patients with the autosomal recessive form of SCID. ADA converts adenosine to inosine or deoxyadenosine to deoxyino-sine. Without this enzyme, there is an accumulation

of adenosine, deoxyadenosine, and deoxyadenosine triphosphate (dATP). This latter compound inhibits ribonucleotide reductase, causing depletion of dezoxyribonucleotide triphosphates and abnormal lymphocyte function. SCID with ADA deficiency may be diagnosed prenatally by amniocentesis.

Secondary deficienciesoccur after full development of the immune system. Some of these are secondary to immunosuppressive therapy, e.g. in tumors, autoimmune diseases, glomerulonephritis, ect. Chronic virus infections and HIV (human immunodeficiency virus) may cause secondary deficiencies. (See AIDS).