Severe combined immunodeficiency (SCID) comprises a group of rare monogenic primary immunodeficiency disorders characterized by a lack of functional peripheral T lymphocytes resulting in early-onset severe respiratory infections and failure to thrive. They are classified according to immunological phenotype into SCID with absence of T cells but presence of B cells (T-B+ SCID) or SCID with absence of both (T-B- SCID). Both of these groups include several forms, with or without natural killer (NK) cells. The disease is Monarch Disease Ontology identifier MONDO_0015974 (severe combined immunodeficiency). Also known as: SCID, combined T and B cell inborn immunodeficiency, severe combined immunodeficiency (disease), severe combined immunodeficiency disease.