Immune deficiency involves a reduced ability of the immune system to function. This may arise from a reduction in components of the immune system. Irradiation can destroy stem cells that give rise to the B and T cells that comprise the immune system. Metabolic disorders, such as adenosine deaminase (ADA) deficiency, may prevent the replication of T cells as well. A very well-known disease, AIDS (acquired immune deficiency syndrome), also causes immune deficiency. HIV (human immunodeficiency virus), the virus that leads to AIDS, reproduces and infects helper T cells, a component of the immune system. The immune system selectively destroys these infected cells. This is a form of self-destruction that eventually leads to an inability of the immune system to function.