ADA is essential for the growth and proper functioning of infection-fighting white blood cells called T and B lymphocytes. Patients who lack this enzyme are, therefore, immune deficient and vulnerable to frequent infections. Injections of PEG-ADA may increase the number of immune cells and reduce infections, but this enzyme replacement therapy is not a definitive cure. In addition, patients may become resistant or allergic to the drug. Gene therapy, in which a normal ADA gene is inserted into the patient's cells, attempts to correcting the underlying cause of disease.