The exact incidence of Severe Combined Immunodeficiency (SCID) is unknown, except that it is recognized as rare in most population groups. It is believed that its incidence is in the neighborhood of 1 in 100,000. This number may vary though, due to misdiagnosis, unreported cases, and the like. SCID is actually a group of disorders. These diseases are either X - linked or autosomal recessive in inheritance. This means both parents must carry the defective chromosome for the child to develop the disease in some cases, and only the mother in others.Severe Combined Immunodeficiency represents a severe defect in T - and B - lymphocyte development (developing white blood cells) resulting in marked susceptibility to severe and complicated infections. (White blood cells are necessary for normal immunity.) The onset of infection usually occurs in the first six months of life. Severe Combined Immunodeficiency is considered to be the most serious of the primary immune disorders. This set of disorders arises because of the inheritance of abnormal genes from one or both parents. The most common form is found in males, and is transmitted via an abnormal X chromosome, provided by the mother. The second most common defect is transmitted to the infant because both parents have contributed an abnormal gene governing the production of a cell enzyme (adenosine deaminase (ADA) or nucleoside phosphorylase) needed for the development of immunity. (This type of inheritance is called autosomal recessive inheritance.) There are additional forms of SCID which have been recognized in the past few years, in a number of cases, the defective genes have been identified. There are three main features of Severe Combined Immunodeficiency, one being, T-helper cells (a kind of white blood cell) function poorly or are absent, or the thymus gland may be small and functions poorly or is absent, and lastly, bone marrow stem cells, from which mature T - and B - lymphocyte...
