Microbiota impact on the immune system. FMT, probiotics, and prebiotics impact the immune system through production of SCFAs such as butyrate, propionate, and acetate. These microbiota-altering therapies likely abrogate and prevent damage to intestinal mucosal integrity resulting from chemotherapy and radiation in patients with hematologic malignancies and those undergoing HSCT. For example, butyrate is a source of nutrition for intestinal epithelial cells, and interacts with dendritic cells through G-protein–coupled receptors (GPCRs) and other mechanisms to induce naive T cells to differentiate into different subtypes of T-regs. T-regs are influenced directly by the local fecal concentrations of SCFAs such as butyrate that can have downstream epigenetic modulatory effects, altering histone acetylation via histone deacetylase (HDAC) and DNA methylation, which may have an effect on colon cancer tumorigenesis and leukemia prognosis.