ADA and severe combined immunodeficiency: Adenosine deaminase-deficient severe combined immunodeficiency disease (ADA-SCID) is an inherited defect that results in the accumulation of enzyme substrates, such as adenosine, 2′-deoxyadenosine and deoxyribonucleotides (dAXP) leading to severe lymphopenia with absence of cellular and humoral immunity and recurrent, severe infections.