Review Question - QID 100461

The patient has difficulty in his immune response to both fungal (Candida) and bacterial (S. pneumonia) infections. This reflects a deficiency in both cell-mediated (T-cell) immunity and humoral (B-cell) immunity. He also has reduced T and B cell counts. Therefore, severe combined immunodeficiency is the most likely cause of this patient's symptoms, and in this case it is the autosomal recessive variant.

In SCID, a deficiency in T-cells impairs both cell-mediated immunity (directly mediated by T-cells) and humoral immunity (deficiency of T-cells means helper function is also impaired). The most common cause of SCID is an X-linked defect leading to an impaired common gamma chain on cytokine receptors. Another cause of SCID is the cellular absence of adenosine deaminase. Both of these impairments lead to a deficiency of T-cells and a subsequent decrease in both cell-mediated and humoral immunity. In SCID that is X-linked, you will have mainly an absence of T-cells. In the autosomal recessive variant SCID, you will have an absence of B-, T-, and NK cells.

Incorrect Answers:
Answer 1: X-linked agammaglobulinemia, also known as Bruton's agammaglobulinemia, results in an absence of B-cells and all immunoglobulins due to a defect in B-cell maturation. T-cell number and function are intact.
Answer 2: Isolated IgA deficiency patients are predisposed to recurrent sinopulmonary and gastrointestinal tract infections. This condition is associated with an anaphylactic response to transfused blood.
Answer 4: DiGeorge syndrome can be remembered by the mnemonic CATCH 22. Cleft lip, Abnormal facies, Thymic aplasia, Cardiac abnormalities, and Hypocalcemia. There is a deletion present on chromosome 22.
Answer 5: MHC class II deficiency is an inherited autosomal recessive trait that has a characteristic deficiency in CD4 T cells. It differs from SCID in that these patients still have T-cells that are able to respond to nonspecific T-cell mitogens.