This thesis discusses the bidirectional relationship between systemic diseases and oral microbiota modification in the immunological aspect. Life expectancy all over the world has been increased in the past few decades. There are more and more elderly people, and most of them have at least one chronic systemic disease. Therefore, the importance of the modified oral microbiota associates with the early diagnosis, treatment, and prognosis of the systemic diseases cannot be overemphasized. Having good oral hygiene is not only beneficial to oral cavity health but also good for the systemic condition.