Research in immunology took a giant step forward with the development and manipulation of the SCID mouse. Lacing an enzyme necessary to fashion a functional immune system of their own, SCID mice-like their human counterparts with Severe Combined Immunodeficiency Disease (Immunodeficiency Diseases)-are helpless not only to fight infection but also to reject transplanted tissue.

In the late 1980s, scientists transformed the SCID mouse into an in vivo model of the human immune system. One group of researchers painstakingly transplanted a human fetal thymus gland and lymph nodes into the adult SCID mouse, then injected them with embryonic human immune cells. Some of these cells traveled to the human thymus, where they matured into T cells; others developed into working B cells and macrophages, circulating through the lymph nodes. A second group of researchers implanted mature human T cells in the SCID mouse. Such systems amount to a living test tube, making it possible to study the effects of drugs and of viruses, including HIV, in an intact mammalian immune system.