An immune deficiency disease may be caused either by an inherited defect in the cells of the immune system or by an environmental agent. In the case of an inherited defect, the resulting disease is referred to as primary immune deficiency disease. According to the World Health Organization, there are more than 200 of these sometimes life-altering or life-threatening inherited defects – some are relatively rare, some are quite common. Primary immune deficiency disease affect many thousands of children and adults in the U.S.
When the damage is caused by an environmental agent, the resulting disease is referred to as secondary immune deficiency disease. Secondary immune deficiencies can be caused by exposure to radiation or toxic chemicals, chemotherapy, malnutrition and burns.
Severe combined immunodeficiency (SCID) represents a group of rare, sometimes fatal, primary immunodeficiencies characterized by one or more serious, life-threatening infections within the first few months of life. They may include pneumonia, meningitis or bloodstream infections. SCID, or “bubble boy disease,” became a household name during the 1970’s with the filming of David Vetter, a boy who lived for 12 years in a plastic, germ-free bubble. The defining feature of SCID, commonly called “bubble boy” disease, is a defect in the specialized white blood cells (B- and T-lymphocytes) that defend us from infection by viruses, bacteria and fungi. Without a functional immune system, SCID patients are susceptible to recurrent infections and may die before the first year of life. Though invasive, new treatments such as bone marrow and stem-cell transplantation save as many as 80% of SCID patients.
All forms of SCID are inherited, with as many as half of SCID cases linked to the X chromosome, passed on by the mother. A mutation in JAK3, located on chromosome 19, can also result in SCID. Defective receptor pathways prevent the proper development of T-lymphocytes, cells that play a key role in identifying invading agents and activating other cells of the immune system.
In another form of SCID, there is a lack of the enzyme adenosine deaminase (ADA), coded for by a gene on chromosome 20. This means that the substrates for this enzyme accumulate in cells. Immature lymphoid cells of the immune system are particularly sensitive to the toxic effects of these unused substrates, so fail to reach maturity. As a result, the immune system of the afflicted individual is severely compromised or completely lacking.
According to Dr. Rebecca Buckley, Chief of Duke University’s Division of Pediatric Allergy and Immunology, “Early diagnosis of SCID is rare because doctors do not routinely perform a blood test on newborns to count white blood cells. Such a blood test could pick up children with SCID as well as those with other serious immune deficiencies that would not be apparent until the child developed an infection. A simple blood test could allow us to treat, and most likely cure, SCID in an infant at a reasonable cost. If found later, less effective treatment can run into the millions. She goes on to say, “…essentially every baby with SCID coud be cured if diagnosed early enough. SCID should be considered a pediatric emergency.”