During the last few decades, immunotherapy is considered to be an important approach to help our immune system to fight various kinds of diseases, such as tumor. Sometimes, it works very well for some types of cancers, for example: bladder cancer, colorectal cancer, breast cancer and lymphoma.

The gut microbiome or gut flora is a vast community of microorganisms such as bacteria, viruses, protozoa, and fungi that inhabit the digestive tract of the human and other animals [1,2]. In the human body, bacterial species colonize into the oral cavity, skin, vagina, and placenta, however, the largest population of microorganisms resides in the intestine.

The immune system developed certain checks and balance to control or inhibit the reactivity against normal cells of the body. Uncontrolled immune responses to the non-self entities such as bacteria, viruses, parasites, or mutated self-antigens can cause an inflammatory reaction and autoimmune diseases.

Cells of the innate and adaptive arm of the immune system including macrophages, natural killer (NK) cells, neutrophils, T cells, and B cells, etc. are crucial for the maintenance of the body’s homeostatic balance and prevention of multiple diseases including cancer.

Since the discovery of escaping mechanism of tumor from negative immune regulation, the paradigm of drug discovery for anti-cancer agents has been dramatically shifted to cancer immunotherapy (e.g., dendritic cell therapy, CAR-T cell therapy, or antibody therapy) by stimulating patient’s immune system to treat cancer.