人體表面皮膚與內部黏膜，是與外界接觸的主要介面，聚集了許多與人體共生的細菌。本文主要探討人類的腸胃道共生菌與免疫系統間的關係。腸胃道共生菌會調節宿主腸道的代謝與免疫功能，和宿主的健康與疾病息息相關。缺乏腸胃道共生菌，會造成腸道免疫系統發育不良，且腸道黏膜的發育與完整性也會受損。目前已知腸胃道共生菌與病原菌兩者具有類似的微生物組成，可被免疫細胞所辨識，進而誘發免疫反應。但腸道的免疫系統，只會對腸道的病原菌產生免疫反應，卻能夠容忍腸胃道的共生菌，對之不產生免疫反應。腸胃道的共生菌叢種類相當多，不同種類的共生菌對免疫系統的影喜也不相同。在腸胃道中存在著不同功能的免疫細胞，包括了促進免疫反應的TH17細胞與液製免疫反應的調節性T細胞(Treg)，前者與腸胃道對抗病原菌感染有關，後者則會維持腸道免疫系統的恆定，避免過度發炎反應。利用特殊的動物模式，發現有些特定的共生菌會促進了TH17細胞的產生，有些共生菌則與調節性T細胞的發生有關。因此，若是某些因素改變了腸胃道共生菌叢的組成，進而造成共生菌叢的失衡(dysbiosis)，則會導致腸胃道免疫系統失調，造成疾病，而這也是形成發炎式大腸炎的可能病因之一。共生菌對免疫系統的作用，雖然主要在腸胃道，但是也會影響到腸胃以外的免疫反應，例如氣喘與自體免疫疾病，也被認為與腸胃道共生菌有關。因此，腸胃道共生菌在人體全身免疫系統的發展上扮演重要的角色。隨著共生菌的研究進展，我們正逐步了解此一複雜的系統如何影響人體免疫恆定與發展。未來這些新知將有助於改善人體免疫系統，並提供許多疾病治療的新方向。

The skin surface and internal mucosa of the human body are the primary sites that expose to the outer environment and accommodate trillions of microorganisms. This article focuses on the intimate relationship between the human immune system and the entire population of gut microflora, collectively known as the gut microbiota. The gut microbiota can regulate the metabolism and gut immunity. Lack of the gut microflora may lead to developmental defects in both the gut immune system and mucosal integrity. Although the gut commensal microorganisms and pathogens share many common components that can induce the host immune response, the gut immune system has evolved to tolerate the gut microflora, but respond well to the invasion of pathogens. The gut microbiota is composed of a variety of species, and distinct species may cause different immune responses. There exist different immune cells with different functions, including pro-inflammatory TH17 cells and anti-inflammatory regulatory T cells (Treg), in the gut. The former prevents the infection of pathogens, whereas the latter maintains the homeostasis of the gut immunity. Using special animal models, several species of commensal microbiota were found to promote the differentiation of TH17 and Treg cells. Imbalance of the gut microbiota, known as dysbiosis, contributes to the development of several diseases, including inflammatory bowel disease. In addition to the gut immunity, the gut microbiota can also affect the extra-intestinal immune system. For example, asthma and autoimmune disease are associated with the dysbiosis of the gut microbiota. Along with the progress of the research on the gut microbiota, we start to understand the underlying mechanisms involved in regulation of the host health and disease by the gut microbiota. Knowledge of this may help to develop better strategy to improve the host immune system and provide a novel opportunity to prevent and treat the intestinal and systemic disorders.