The invention relates generally to compositions and methods of using transgenic non-human animals expressing human SIRPα that are engrafted with a human hematopoietic system. In various embodiments, the human hematopoietic system engrafted, human SIRPα transgenic non-human animals of the invention are useful as systems for the in vivo evaluation of the growth and differentiation of hematopoietic and immune cells, for the in vivo assessment of an immune response, for the in vivo evaluation of vaccines and vaccination regimens, for in vivo production and collection of immune mediators, including human antibodies, and for use in testing the effect of agents that modulate hematopoietic and immune cell function.