A genetically modified immunodeficient non-human animal whose genome includes a genetic modification that renders the non-human animal deficient in macrophages and/or macrophage anti-human red blood cell activity so as to prolong the survival of human red blood cells when administered into said non-human animal is provided according to aspects of the present invention. Methods of assaying effects of putative therapeutic agents in such a genetically modified immunodeficient non-human animal are provided by the present invention.