- Severe combined immunodeficiency (SCID) is a genetic disorder characterized by the absence of both T cells and B cells, resulting in impaired cell-mediated and humoral immunity. - SCID can be caused by defects in cytokine receptors, genes involved in lymphocyte development and differentiation, or enzymes involved in purine metabolism. - Patients with SCID present with recurrent and persistent infections by various pathogens. The diagnosis of SCID is suggested by very low lymphocyte counts and confirmed by functional tests of lymphocytes. Bone marrow transplantation or gene therapy can cure SCID in many cases.