A huge number of indigenous commensal bacteria reside in the intestines of humans and animals. However, the host animals do not unconditionally accept gut microbiota. In order to contain gut microbiota by secreting immunoglobulin A, the intestine is equipped with the intestinal immune system, literally the largest peripheral lymphoid tissue in the body where 60 to 70% of peripheral immune cells are accumulated. On the other hand, the gut microbiota greatly impact the host physiology and pathology. Normal development of the host immune system relies on interaction with the gut microbiota. In addition, abnormal gut microbiota, or dysbiosis, is known to be associated with various disease statuses including autoimmune diseases. Understanding of the causal relationship between the pathophysiology of these diseases and dysbiosis is still limited, but verification experiments using animal models have been clarifying that gut microbiota is an important regulatory factor the pathogenesis and progression of these diseases.
Keywords: Gut microbiota; Intestinal immune system; Multiple sclerosis; Type 1 diabetes mellitus.