Severe combined immunodeficiency (SCID) is a group of heterogeneous disorders with similar clinical pictures but variable defects resulting in severe dysfunction of T and B cells.

Disease occurs in 1 per 100 000 births, and slightly less than half of these are due to the X-linked form of SCID, which affects only males. The majority of cases are autosomal recessive in their inheritance. The X-linked form of SCID is due to a mutation of the common gamma chain of interleukin receptors for IL-2, 4, 7, 9, and 15. There are few to no T cells (CD3) and natural killer (NK) cells. Variable levels of B cells make nonfunctional antibodies. Two to three percent of SCID cases are caused by deficiency in the DNA-repair enzyme Artemis, resulting from mutations in DCLRE1C. Other forms of SCID have varying immunologic abnormalities, and diagnosis is made on direct gene analysis.

Failure to thrive (poor feeding and low weight gain), chronic diarrhea and infections (especially pneumonias, otitis media, and skin infections), mucocutaneous candidiasis, and skin eruptions mark the course of disease. Graft-versus-host disease (GVHD) from maternal cell engraftment occurs in about half of patients.

The average age of symptom onset is 2 months. Without therapy, death from infection usually occurs in the first 2 years of life.