Adenosine deaminase deficiency

Adenosine deaminase (ADA) deficiency is a disorder that affects the immune system. Specifically, ADA deficiency impairs the development and function of immune cells called lymphocytes. Lymphocytes are white blood cells that help the body fight infections. As a result, people with ADA deficiency often develop pneumonia, chronic diarrhea, and widespread skin rashes. Additional signs and symptoms of ADA deficiency include slow growth and developmental delays.

About 80 percent of individuals with ADA deficiency also have severe combined immunodeficiency (SCID). People with SCID lack virtually all immune protection from bacteria, viruses, and fungi. They are prone to repeated and persistent infections that can be 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. People with ADA deficiency with SCID (ADA-SCID) typically develop health problems within the first 6 months of life. Without treatment, these babies usually do not survive past age 2.

About 15 to 20 percent of people with ADA deficiency develop health problems that begin between 1 and 10 years of age (delayed onset) or in adulthood (late onset). In people with this form of ADA deficiency (known as delayed or late-onset combined immunodeficiency or ADA-CID), the immune deficiency tends to be less severe than in people with ADA-SCID. People with ADA-CID typically have recurrent upper respiratory and ear infections. Over time, affected individuals may develop chronic lung damage, malnutrition, and other health problems.

In some individuals, ADA deficiency only impacts red blood cells. Since white blood cells are not affected, these individuals have normal immune systems. This form of the condition is known as partial ADA deficiency. Individuals with this form do not have any health problems related to the condition. They often only find out they have ADA deficiency when they undergo testing because of an affected relative or during a normal health screening.