Immunodeficiency diseases are characterized by an undue susceptibility to infection and/or development of tumours. Immunodeficiency can affect one or more components of the immune system, including T-, B-, and natural killer (NK)-lymphocytes, as well as phaogocytic cells and complement proteins. Immunodeficiency can be reflecting an inherent defect in immune cells and these are usually congenital (genetically transmitted) or secondary, which is an usually acquired defect. An example of a primary immune deficiency is Severe Combined Immunodeficiency (SCID) characterized by impairment of both B and T cell function, due to mutations in the RAG-1 or RAG-2 genes, in which affected individuals rarely survive beyond 1 year of life without treatment. The best known secondary immunodeficiency is AIDS, which is due to infection by the Human Immunodeficiency Virus (HIV) and is characterized by severe depletion of CD4 T cells.