Chimeric immunocompromised mammals and their use

Chimeric hosts are provided by introducing into an immunocompromised mammalian host xenogeneic, usually human, tissue under conditions where the xenogeneic tissue may both grow and interact, in such a way that undifferentiated ("stem") cells may be observed to differentiate and to proliferate. For example, tissues from hematopoietic organs are employed where tissue from different organs may be used which allow for differentiation and proliferation of the xenogeneic hematopoietic stem cells. Into SCID mice may be introduced fetal tissue, fetal liver tissue providing stem cells, thymus tissue, where the thymus grows into a competent thymus organ, and lymph node tissue, where the lymph node tissue grows into a competent lymph node, and the stem cells are processed to produce functional human B- and T-cells.