Over the past ten years there has been a marked increase in recognition of the interplay between the intestinal microbiome and the hematopoietic system. Despite their apparent distance in the body, a large literature now supports the relevance of the normal intestinal microbiota to steady state blood production, affecting both hematopoietic stem and progenitor cells and differentiated immune cells. Microbial metabolites enter the circulation where they can trigger cytokine signaling …