Within the gastrointestinal tract of the human body thrive trillions of bacteria, comprising what is known as the microbiota (Slack et al. 2009, Figure 1). The microbiota can be defined as the combination of microorganisms, consisting primarily of bacteria, living simultaneously in a location (Round et al. 2009). Thousands of years of evolution have contributed to the relationship that is observed between human intestinal bacteria and the adaptive immune system. Beginning with the initial divergence of vertebrates, humans have acquired the ability to establish and maintain a microbiota, which can be altered in response to various factors. Contrary to the common belief that all intestinal bacteria are pathogenic, recent evidence suggests that the bacteria residing in the human gut actually play a crucial role in human adaptive immunological function.
Vertebrate Immune System Divergence
The ability to evade pathogens using various immunological strategies is something that all multicellular organisms are capable of (Boehm 2012). Before vertebrates existed, invertebrates relied exclusively on innate immunity as a line of defense. Near the beginning of this divergence, the lymphocyte cell emerged, marking the evolution of adaptive immunity (Slack et al. 2009). A lymphocyte is a type of white blood cell that undergoes self-renewal and plays a role in humoral and cell-mediated immune responses. Yet, the question remained as to what the advantage of evolving adaptive immunity amongst vertebrates actually was. Research has determined that hundreds of resident species of bacteria reside in the guts of vertebrate organisms, while less than ten bacterial species are usually found at any given time in the guts of invertebrates (McFall-Ngai 2007). This suggested that the lymphocyte-rich adaptive immune system found in vertebrates has evolved to manage various mutualistic species of bacteria within the gastrointestinal system (McFall-Ngai 2007).
While the functions of the innate and adaptive immune system overlap in some aspects, the adaptive immune system differs in its mechanisms that maintain a mutualism between bacteria and vertebrate hosts. As was previously mentioned, T and B lymphocytes distinguish adaptive from innate immunity. It’s crucial that each lymphocyte contains one unique antigen receptor, which allows the individual lymphocyte to generate a memory response to the antigen it has encountered previously (Boehm 2012). Only when innate immunity is compromised do intestinal bacteria produce chemical signals from macrophages to stimulate an adaptive, or acquired, immune response. This is important in fighting off unwelcome pathogens (Hooper et al. 2010). With the establishment of the adaptive immune system in vertebrates, it comes as no surprise that humans also possess this type of immunity.
Human Microbiota Establishment
Several important effects of the microbiota on the human immune system have been observed by studies using the...