Persons with severe combined immune deficiency (SCID) are totally unable to mount an immune response to antigens. Both the B and T lymphocytes are affected. The disease arises from an inherited lack of a degradative enzyme, adenosine deaminase (ADA). The reaction shown here illustrates the pathways affected.

Lack of ADA allows dATP to accumulate. High dATP levels inhibit production of the other dNTPs needed for DNA replication due to its effect on the regulation of ribonucleotide reductase. White blood cells, which must proliferate for an immune response to occur, and which have abundant salvage enzymes for making dATP, are most affected by lack of the enzyme. They are unable to proliferate, a necessary step for antibody production.