Adenosine deaminase (ADA deficiency) is an inherited disorder that damages the immune system and causes severe combined immunodeficiency (SCID). Children with SCID lack virtually all immune protection from bacteria, viruses, and fungi. They are prone to repeated and persistent infections that can be very serious or life-threatening. These infections are often caused by “opportunistic” organisms that ordinarily do not cause illness in people with a normal immune system. Infants with SCID typically suffer from pneumonia, chronic diarrhea and widespread skin rashes. They also grow much more slowly than healthy children. Some of these children also have neurological problems, including developmental delays, movement disorders and hearing loss. If not treated in a way that restores immune function, children with SCID usually live only a year or two.
Most individuals with ADA deficiency are diagnosed with SCID in the first 6 months of life. In about 10 percent to 15 percent of cases, onset of immune deficiency is delayed to between 6 and 24 months of age (delayed onset) or even until adulthood (late onset). Immune deficiency in these later-onset cases tends to be less severe, primarily resulting in recurrent upper respiratory and ear infections. Over time, affected individuals may develop chronic lung damage, allergies and other health problems