Severe combined immunodeficiency (SCID) is a genetic disorder of the immune system that occurs when the T-lymphocyte system does not function properly. SCID is the most severe type of primary immune deficiency diseases. Primary immunodeficiencies are disorders that occur because part of the body's immune system does not function properly. Unlike secondary immunodeficiencies, which are caused by external factors like viruses or chemotherapy, primary immunodeficiencies are caused by problems that originate within the patient's own body. SCID is usually diagnosed during childhood. A milder form of T-lymphocyte dysfunction causes combined immunodeficiency, which is typically diagnosed in adults.
The immune system protects the body from harmful antigens (foreign substances that enter the body) like bacteria, viruses, fungi, or parasites. A specialized tissue inside bones called marrow produces immune system cells, which are also called stem cells. These stem cells then develop into B-lymphocytes, T-lymphocytes and natural killer (NK)-lymphocytes and phagocytes. All of these cells produce proteins that help them detect antigens that enter the body. The cells and proteins are spread throughout the body to protect the body from diseases and infections.
There are three types of T-lymphocytes:
T-helper cells, cytotoxic T-cells, and regulatory (T-reg or T-suppressor) T-cells. When an antigen enters the body, T-helper cells direct B-lymphocytes (B-cells) to make antibodies against it. These antibodies then bind to the foreign substance, which is called an antigen. This binding tells the T-cytotoxic cells to destroy the unwanted antigen. T-reg cells direct the resolution of the immune system response, which means they "turn off" the immune response.
The defining characteristic of SCID is always a severe defect in T-cell production and function. Patients also have dysfunctional B-lymphocytes, which may or may not be caused by the dysfunctional T-cells. Some genetic types of SCID also cause a shortage of NK-cell production as well. Consequently, SCID patients produce little or no antibodies, and they are extremely vulnerable to developing opportunistic infections (infections that occur in individuals with weakened immune systems).
Unless these problems in the immune system are corrected, the child will die from severe opportunistic infections by the age of one or two. Most patients are diagnosed when they are six and a half months old.
There are several types of SCID, including autosomal recessive severe combined immunodeficiency, X-linked recessive severe combined immunodeficiency, adenosine deaminase deficiency (ADA), bare lymphocyte syndrome, severe combined immunodeficiency with leukopenia (reticular dysgenesis), and Swiss-type agammaglobulinemia.
SCID is genetically inherited as either an X-linked (gene on the X chromosome) or autosomal recessive (two copies of a single mutated gene) trait. Each type is caused by a different genetic defect in the patient's DNA. However, the genetic defect has not been discovered for all forms of the disease. Since SCID patients have impaired immune systems, they are vulnerable to diseases and infections. Patients commonly suffer from pneumonia, meningitis, and/or blood system infections.
Without a bone marrow transplant, a child with SCID is at risk of developing a severe or deadly infection. While waiting for a bone marrow transplant, patients are treated with intravenous immune globulin (IVIG), which helps the immune system fight off illness. Antibiotics, antifungals and antivirals are given to treat infections associated with SCID. Without treatment, most patients do not live beyond one year from birth.
SCID is considered a rare disorder, but the exact incidence of SCID is unknown. Researchers estimate that about 1 out of 1,000,000 people have the disease. Overall, the male-to-female ratio is 3:1 because some forms of SCID are X-linked, while other forms of SCID are autosomal recessive.
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