Which immunodeficiency is characterized by severe combined immunodeficiency with defective thymic T-cell development, often requiring stem cell transplantation for survival?

• A. Wiskott-Aldrich syndrome
• B. SCID (severe combined immunodeficiency)
• C. X-linked agammaglobulinemia
• D. Hyper-IgM syndrome

Answer: (B)

Severe combined immunodeficiency is defined by a profound defect in T-cell development in the thymus, which cripples cellular immunity and, as a consequence, impairs humoral responses as well. When thymic T-cell maturation is blocked, you lose mature CD4+ and CD8+ T cells, and without functional T-helper activity, B cells don’t receive the signals they need to generate proper antibody responses. Infants with this condition typically present early with recurrent, severe infections, failure to thrive, chronic diarrhea, and thrush. Because the immune system is so severely compromised, definitive treatment often requires hematopoietic stem cell transplantation to reconstitute immune function; gene therapies are being used for some genetic forms (like ADA-SCID or IL-2R gamma chain defects). Other immunodeficiencies differ in which cells are affected: Wiskott-Aldrich has eczema, thrombocytopenia, and immunodeficiency; X-linked agammaglobulinemia features absent or very few B cells and low immunoglobulins with relatively normal T cells; Hyper-IgM syndrome involves defective class switching with high or normal IgM but low IgG/IgA, and normal or near-normal T-cell numbers. The hallmark of this scenario is the defective thymic T-cell development leading to a life-threatening, early-onset combined immunodeficiency that often needs stem cell transplantation for survival.