Severe combined immunodeficiency syndromes (SCID) are group of rare congenital genetic diseases that result in combined T lymphocyte and B lymphocyte deficencies.
These syndromes are cause by defective hematopoietic progenitor cells which are the precursors of both B- and T-cells. This results in a severe reduction in developing thymocytes in the thymus and consequently thymic atrophy. A number of genetic defects can cause SCID.
Few examples are IL-7 receptor deficiency, common gamma chain deficiency, and Recombination activating gene deficiency.