Severe combined immunodeficiency disease: A combined deficiency of the immune system in which its two major weapons, antibodies and T cells, are genetically missing or disabled. Severe combined immunodeficiency disease (SCID) is rare. The chances of a child being born with SCID are about one in 500,000 births. Until recent years, it was always fatal.
There are a number of different causes of SCID. Each is caused by a different genetic defect, and each develops along a different pathway:
- X-linked SCID, the most common type, a genetic flaw damages molecules that allow T cells and B cells to receive signals from crucial growth factors.
- ADA deficiency results from the lack of an enzyme called adenosine deaminase (ADA) that helps cells, especially immune cells, to get rid of toxic byproducts. Without ADA, poisons build up and kill the lymphocytes.
- Purine nucleoside phosphorylase (PNP) deficiency results from a similar enzyme problem, but B cells are less affected and the immunodeficiency is less severe, although affected patients may have other problems (neurologic).
- Yet another type of SCID is known as MHC class II deficiency or bare lymphocyte syndrome. MHC molecules are specialized proteins found on the surface of body cells and play an important role in bone marrow transplantation. Class II MHC molecules, which appear on many immune cells, allow B cells and other immune cells to recognize, interact with, and activate T cells. Without this B cell/T cell communication, the immune defenses are compromised.
Whatever the underlying problem that causes SCID, the consequences are nearly always the same. The child lacks almost all immune defenses, develops life-threatening infections, and needs major treatment to survive beyond infancy. Although the specifics vary from case to case, these children are vulnerable to serious infections caused by bacteria, as is typical with a B cell deficiency, and also by viruses and opportunistic germs, as is the case with a T cell deficiency.
Usually by the time a baby is three months old, he or she (because many cases of SCID are X-linked, SCID is more common in boys than in girls) is likely to have persistent thrush or extensive diaper rash. Weakened by chronic diarrhea, the baby may stop growing and gaining weight. Some children develop a sharp, persistent cough with Pneumocystis pneumonia, blood disorders, or chronic hepatitis. Meningitis and blood poisoning pose a constant threat.
Viruses that are not harmful in children with normal immunity can pose a serious danger. For example, the virus that causes chicken pox (varicella) can trigger a severe infection in the lungs and the brain of SCID patients. Other threats come from the viruses that cause cold sores (herpes simplex) and measles (rubeola).
The treatment of SCID constitutes a medical emergency. The immediate concern is to bring any current infections under control, and to strengthen the baby's weakened condition with adequate nutrition. IVIG may help to bolster the immune responses. A lasting remedy, however, requires a more drastic approach. A bone marrow transplant from a matched donor or parent is arranged as quickly as possible. Children whose SCID is due to ADA deficiency have another alternative. Injections of PEG-ADA will protect them against recurrent infections, allow them to control ordinary childhood infections such as chicken pox, and make it feasible for them to lead nearly normal lives.