Severe combined immunodeficiency (SCID) is an

• A. Immunodeficiency with decreased B cells and neutrophils
• B. Immunodeficiency with lymphocytopenia and eosinophilia
• C. Immunodeficiency with decreased or dysfunctional T and B cells
• D. Immunodeficiency with decreased lymphocytes and decreased complement concentrations

Answer: (C)

SCID represents a failure of the adaptive immune system in which both arms of the T- and B-lymphocyte responses are severely impaired. When T cells are markedly reduced or dysfunctional, helper signals for B cells are lost, so antibody production is also greatly diminished. This simultaneous deficiency of T and B cell function is the hallmark of SCID, often due to various genetic defects that disrupt lymphocyte development or signaling. Patients typically present very early with life-threatening infections because neither cellular nor humoral immunity can mount adequate defense. The best description among the options is an immunodeficiency with decreased or dysfunctional T and B cells, which directly captures the dual impairment at the heart of SCID. Other patterns—such as isolated neutropenia, eosinophilia with lymphocytopenia, or reduced lymphocytes plus reduced complement—do not reflect this combined T- and B-cell failure.