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Ataxia telangiectasia (AT) is an autosomal recessive disorder characterised by cerebellar ataxia, telangiectasia, immune defects, and a predisposition to malignancy. Chromosomal breakage is a feature. AT cells are abnormally sensitive to cell kill by ionising radiation and abnormally resistant to inhibition of DNA synthesis by ionising radiation. The responsible gene, 'ataxia telangiectasia mutated' (ATM) plays a crucial role in a signal transduction pathway, regulating the cell cycle, and in preventing damaged DNA from being reproduced. This rare genetic disorder manifests itself during childhood. The illness is progressive and most individuals die in their second or third decade of life due to infections or cancer. AT is difficult to diagnose due to its rarity and clinical heterogeneity. Both a physical examination and several laboratory tests are necessary for establishing its proper diagnosis.

Severe congenital immunodeficiency diseases occur in approximately 1:10,000 newborns, in Western Europe and North America. They are characterised by recurrent infections, mostly caused by opportunistic micro-organisms, and autoimmune phenomena. In many patients the immune deficiency occurs as part of a syndrome. An immunological dysfunction may be caused by total absence, strong reduction or dysfunction of one or more cellular elements or of cell-associated or cellular secretion products. It is convenient to divide the immune system into a cellular compartment (with specific T and B cells, and non-specific natural killer cells and myelomonocytes), a compartment for cellular interactions (adhesion, costimulation and communication through cytokines) and an aspecific opsonization compartment. In cases of deficient humoral immunity treatment options are antimicrobial drugs and substitution therapy with immunoglobulins, in cases of phagocytic dysfunction the application of intracellularly active antibiotics and in cases of disturbances of cellular immunity allogeneic transplantation of haematopoietic stem cells and in the future perhaps the repair of the genetic defect by somatic gene therapy.