Severe combined immunodeficiency

Definition

Severe combined immunodeficiency (SCID) is the most serious human immunodeficiency disorder(s). It is a group of congenital disorders in which both the humoral part of the patient's immune system and the cells involved in immune responses fail to work properly. Children with SCID are vulnerable to recurrent severe infections, retarded growth, and early death.

Description

SCID is thought to affect between one in every 100,000 persons, and one in every 500,000 infants. 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 in an autosomal recessive pattern.
• Bare lymphocyte syndrome. In this form of SCID, the white blood cells (lymphocytes) in the baby's blood are missing certain proteins. Without these proteins, the lymphocytes cannot 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.

In order to understand why SCID is considered the most severe immunodeficiency disorder, it is helpful to have an outline of the parts of the human immune system. It has three parts: cellular, humoral, and nonspecific. The cellular and humoral parts of the system 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.

Causes & symptoms

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 cells are 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 and pulmonary infections, including pneumocystis pneumonia. The child loses weight, becomes very weak, and eventually dies from an opportunistic infection.

Diagnosis

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.

Treatment

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 standard treatments for SCID.

Investigational treatments

In 1990, the Food and Drug Administration (FDA) approved PEG- ADA, an orphan drug (not available in US 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. In gene therapy, the children receive periodic infusions of their own T cells corrected with a gene for ADA that has been implanted in an activated virus.

Prognosis

As of 1998, there is no cure for SCID. Most untreated patients die before age two.

Prevention

Genetic counseling is recommended for parents of a child with SCID.

Key Terms

Adenosine deaminase (ADA)

An enzyme that is lacking in a specific type of SCID. Children with an ADA deficiency have low levels of both B and T cells.

Antigens

A substance that usually causes the formation of an antibody. A foreign invaders in the body.

Autosomal recessive inheritance

A pattern of inheritance of a recessive gene where, among other things, both parents may not show symptoms.

B cell

A type of lymphocyte or white blood cell that is derived from precursor cells in the bone marrow.

Congenital

Present at the time of birth. Most forms of SCID are hereditary as well as congenital.

Gene therapy

An experimental treatment for SCID that consists of implanting a gene for ADA into an activated virus and merging it with some of the patient's own T cells. The corrected T cells are infused back into the patient every few months.

Humoral

Pertaining to or derived from a body fluid. The humoral part of the immune system includes antibodies and immunoglobulins in blood serum.

Lymphocyte

A type of white blood cell that is important in the formation of antibodies.

Orphan drug

A drug that is known to be useful in treatment but lacks sufficient funding for further research and development.

PEG-ADA

An orphan drug that is useful in treating SCID related to ADA deficiency.

T cells

Lymphocytes that originate in the thymus gland. T cells regulate the immune system's response to infections. The thymus gland is small or underdeveloped in children with SCID.

Thrush

A disease of the mouth caused by a yeast, .

Further Reading

For Your Information

Books

• Abbas, Abul K., et al. Cellular and Molecular Immunology. Philadelphia: W. B. Saunders Company, 1997.
• "Immunology: Immunodeficiency Diseases." In The Merck Manual of Diagnosis and Therapy, edited by Robert Berkow, et al. Rahway, NJ: Merck Research Laboratories, 1992.
• Physicians' Guide to Rare Diseases, edited by Jess G. Thoene. Montvale, NJ: Dowden Publishing Company, Inc., 1995.
• Roitt, Ivan M. Roitt's Essential Immunology. Oxford, UK: Blackwell Science Ltd., 1997.

  Organizations

• Immune Deficiency Foundation. 25 West Chesapeake Avenue, Suite 206, Towson, MD 21204. (410) 321-6647.
• National Organization for Rare Disorders (NORD). P.O. Box 8923, New Fairfield, CT 06812-8923. (800) 999-NORD. (203) 746-6927 (TDD).

Gale Encyclopedia of Medicine. Gale Research, 1999.