Review Question - QID 100460

Recurrent infections with fungal and bacterial pathogens are likely the result of defective T and B cell immunity. Severe combined immunodeficiency syndrome (SCID) is the only disorder of both B and T cell lineages present among the answer choices. It also results in the absence of a thymic shadow.

Impaired humoral and cell-mediated immunity are hallmarks of severe combined immunodeficiency. The most common cause of SCID is a X-linked mutation of the IL-2 receptor. Other deficiencies include adenosine deaminase deficiency. The absence of this enzyme leads to the build-up of deoxyadenosine within cells. For lymphocytes (notably T-cells), this build-up is toxic; the result is a deficiency in T-cells. The result is a great decrease in the number of T-cells and a subsequent decrease in humoral immunity which relies on T-helper cells for activation

Uribe and Weinberg describe defects in cytokine pathways. They note that the IL-2 receptor is expressed by T cells after antigenic stimulation and when engaged by IL-2 induces proliferation. Thus, to become activated a T cell must have contact with an antigen and a second signal in the form of IL-2. IL-2 therefore is a major growth factor for mature T lymphocytes but not for immature thymocytes.

Cooper et al. discuss primary immunodeficiencies. They note that SCID results in severe opportunistic infections, chronic diarrhea, and failure to thrive in infancy. Additionally, infants are at risk for graft-versus-host disease due to their ability to reject foreign tissue. Thus maternal T cells that cross into fetal circulation pose a threat to the native organs of the fetus.

Illustration A is a chest X-ray showing a normal thymus gland in a young child. Patients with SCID lack the thymic shadow circled in the picture.

Incorrect answers:
Answer 2: X-linked agammaglobulinemia does not result in the loss of a thymic shadow or recurrent fungal infections.
Answer 3: DiGeorge syndrome is thymic aplasia due to a failure to develop the 3rd and 4th pharyngeal pouches. This results in hypocalcemia, recurrent viral/fungal infections, and congenital heart defects.
Answer 4: Isolated IgA deficiency presents as sinus and lung infections, milk allergies, and diarrhea. There is no loss of the thymic shadow.
Answer 5: Common variable immunodeficiency is a defect in B-cell maturation typically acquired later in life. There is no loss of thymic shadow.