Scid Health Dictionary

Disorders in which there is a failure of the immune system’s defences to fight infection and tumours. They may be due to an inherited or a congenital defect or may be the result of acquired disease. The result is persistent or recurrent infection, including those with organisms that would not ordinarily cause disease, and an undue susceptibility to certain forms of cancer. The infections in people with immunodeficiency disorders are sometimes called opportunistic infections; examples include pneumocystic pneumonia, fungal infections, and widespread herpes simplex infections.

Congenital or inherited deficiencies can occur in either of the 2 prongs of the adaptive immune system: humoral or cellular. Deficiencies of the humoral system include hypogammaglobulinaemia and agammaglobulinaemia. The former may cause few or no symptoms, depending on the severity of the deficiency, but agammaglobulinaemia can be fatal if not treated with immunoglobulin. Congenital deficiencies of T-lymphocytes may lead to problems such as persistent and widespread candidiasis (thrush). A combined deficiency of both humoral and cellular components of the immune system, called severe combined immunodeficiency (SCID), is usually fatal in the 1st year of life unless treatment can be given by bone marrow transplant.

Acquired immunodeficiency may be due either to disease processes (such as infection with HIV, which leads to AIDS) or damage to the immune system as a result of its suppression by drugs. Severe malnutrition and many cancers can also cause immunodeficiency. Mild immunodeficiency arises through a natural decline in immune defences with age.... immunodeficiency disorders

(ADA deficiency) a genetic disorder affecting about one baby in 25,000 and characterized by a defect in adenosine deaminase (ADA), an enzyme that is involved in purine metabolism. Deficiency of this enzyme results in selective damage to the antibody-producing lymphocytes; this in turn leads to a condition known as *severe combined immune deficiency (SCID), in which the affected baby has no resistance to infection and must be entirely isolated from birth. Such children have only about a 50% chance of surviving for six months. See also gene therapy.... adenosine deaminase deficiency

the cell from which all classes of blood cells are derived. It cannot be identified microscopically, but can be defined by the presence of a combination of cell-surface proteins. It can be demonstrated by *tissue culture of the blood-forming tissue of the bone marrow, as well as by growth of human haemopoietic cells in immunodeficient mice strains, such as non-obese diabetic/severe combined immunodeficient (NOD/SCID). See also haemopoiesis.... haemopoietic stem cell

(SCID) a rare disorder that usually manifests itself within the first three months of life by severe bacterial, fungal, and viral infection and *failure to thrive. It is due to reduced numbers of T and B *lymphocytes – white blood cells necessary for fighting infection. Some cases are caused by *adenosine deaminase deficiency. The only treatment currently available is a bone-marrow transplant, but *gene therapy offers hope for the future.... severe combined immune deficiency