Severe combined immunodeficiency (SCID) is the most serious human immunodeficiency disorder(s). It is a group of disorders existing from the time of birth (congenital) in which the patient's immune system, and the cells involved in immune responses fail to work properly. Children with SCID are vulnerable to recurrent severe infections, retarded growth, and early death. It is thought to affect between one in every 100,000 persons, and one in every 500,000 infants.

In order to understand why SCID is considered the most severe immunodeficiency disorder, it is helpful to have an outline of the human immune system which has three parts: cellular, humoral, and nonspecific. The cellular and humoral parts are both needed to fight infections--they recognize disease agents and attack them. The cellular system is composed of many classes of T-lymphocytes (white blood cells that detect foreign invaders called antigens). The humoral system is...