Severe combined immunodeficiency

Severe combined immunodeficiency (SCID) is the most serious human immunodeficiency disorder(s). It is a group of disorders existing from the time of birth(congenital) in which the patient's immune system, and the cells involved inimmune responses fail to work properly. Children with SCID are vulnerable torecurrent severe infections, retarded growth, and early death. It is thoughtto affect between one in every 100,000 persons, and one in every 500,000 infants.

In order to understand why SCID is considered the most severe immunodeficiency disorder, it is helpful to have an outline of the human immune system whichhas three parts: cellular, humoral, and nonspecific. The cellular and humoral parts are both needed to fight infections--they recognize disease agents and attack them. The cellular system is composed of many classes of T-lymphocytes (white blood cells that detect foreign invaders called antigens). The humoral system is made up of B-cells, which are the only cells in the body that make antibodies. In SCID, neither the cellular nor the humoral part of the immune system is working properly.

Several different immune system disorders are currently grouped under SCID:

  • Swiss-type agammaglobulinemia. This was the first type of SCID discovered, in Switzerland in the 1950s.
  • Adenosine deaminase deficiency (ADA). About 50% of SCID cases are of this type. ADA deficiency leads to low levels of B and T cells in the child's immune system.
  • Autosomal recessive. About 40% of SCID cases are inherited from the parents.
  • Bare lymphocyte syndrome. In this form of SCID, the white blood cells (lymphocytes) inthe baby's blood are missing certain proteins without which the lymphocytescannot activate the T cells in the immune system.
  • SCID with leukopenia. Children with this form of SCID are lacking a type of white blood cell called a granulocyte.

SCID is an inherited disorder. There are two ways in which a developing fetus' immune system can fail to develop normally. In the first type of genetic problem, both B and T cells are defective. In the second type, only the T cellsare abnormal, but their defect affects the functioning of the B cells.

For the first few months of life, a child with SCID is protected by antibodies in the mother's blood. As early as three months of age, however, the SCID child begins to suffer from mouth infections (thrush), chronic diarrhea, otitis media (ear infections), and pulmonary (associated with the lungs) infections, including pneumocystis pneumonia. The child loses weight, becomes very weak, and eventually dies from an opportunistic infection--one which takes advantage of the patient's weakened state.

SCID is diagnosed by the typing of T and B cells in the child's blood. B cells can be detected by immunofluorescence tests for surface markers (unique proteins) on the cells. T cells can be identified in tissue sections (samples) using enzyme-labeled antibodies.

Patients with SCID can be treated with antibiotics and immune serum to protect them from infections, but these treatments cannot cure the disorder. Bone marrow transplants are currently regarded as one of the few effective standardtreatments for SCID.

In 1990, the Food and Drug Administration (FDA) approved PEG-ADA, an orphan drug (not available in United States but available elsewhere), for the treatment of SCID. PEG-ADA, which is also called pegademase bovine, works by replacing the ADA deficiency in children with this form of SCID. Children who receive weekly injections of PEG-ADA appear to have normal immune functions restored. Another treatment that is still in the experimental stage is gene therapy.

As of 1998, there is no cure for SCID. Most untreated patients die before agetwo years. Genetic counseling is recommended for parents of a child with SCID.

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