The loss of infection-fighting cells results in the signs and symptoms of SCID. The number of lymphocytes in other lymphoid tissues is also greatly reduced. These cells die before they can mature to help fight infection. Immature lymphocytes in the thymus are particularly vulnerable to a toxic buildup of deoxyadenosine. Mutations in the ADA gene reduce or eliminate the activity of adenosine deaminase and allow the buildup of deoxyadenosine to levels that are toxic to lymphocytes. Adenosine deaminase converts deoxyadenosine, which can be toxic to lymphocytes, to another molecule called deoxyinosine that is not harmful. The function of the adenosine deaminase enzyme is to eliminate a molecule called deoxyadenosine, which is generated when DNA is broken down. Lymphocytes in the blood and in lymphoid tissues make up the immune system. Lymphocytes are produced in specialized lymphoid tissues including the thymus, which is a gland located behind the breastbone, and lymph nodes, which are found throughout the body. These cells protect the body against potentially harmful invaders, such as bacteria and viruses, by making immune proteins called antibodies or by directly attacking infected cells. This enzyme is found throughout the body but is most active in specialized white blood cells called lymphocytes. This gene provides instructions for producing the enzyme adenosine deaminase. Over time, affected individuals may develop chronic lung damage, malnutrition, and other health problems.Īdenosine deaminase deficiency is caused by mutations in the ADA gene. Immune deficiency in these later-onset cases tends to be less severe, causing primarily recurrent upper respiratory and ear infections. 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). Without treatment, these babies usually do not survive past age 2. Most individuals with ADA deficiency are diagnosed with SCID in the first 6 months of life. Affected children also grow much more slowly than healthy children and some have developmental delay. The main symptoms of ADA deficiency are pneumonia, chronic diarrhea, and widespread skin rashes. These infections are often caused by "opportunistic" organisms that ordinarily do not cause illness in people with a normal immune system. They are prone to repeated and persistent infections that can be very serious or life-threatening. People with SCID lack virtually all immune protection from bacteria, viruses, and fungi. Adenosine deaminase (ADA) deficiency is an inherited disorder that damages the immune system and causes severe combined immunodeficiency (SCID).
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