Abstract: The ecological balance of intestinal microbiota plays an important role in digestion, absorption,  metabolism, immunity and protection against pathogens. Intestinal microbiota can not only regulate the innate immunity of the body, but also can stimulate the immune response by the bacteria itself and its metabolites. The imbalance intestinal microbiota may lead to abnormal immune mechanism, and then participate in the occurrence and development of the tumor, especially colorectal cancer.

Key words: Neoplasms, Immunity, Intestinal microbiota