Adenosine deaminase deficiency is seen in the following:

The most common cause of autosomal recessive severe combined immunodeficiency(SCID) is the deficiency of enzyme adenosine deaminase (ADA)Defect lies in lymphoid stem cells, resulting in failure of both T and B cellsDeficiency of ADA leads to accumulation of deoxyadenosine & its derivatives, which are toxic to immature lymphocytes, especially those of T lineage, and this may account for the marked decrease in T lymphocyte number in severe cases(ref: Harrison's 18/e p2700)