ⓘ Microbial symbiosis and immunity
Long-term close-knit interactions between symbiotic microbes and their host can alter host immune system responses to other microorganisms, including pathogens, and are required to maintain proper homeostasis. The immune system is a host defense system consisting of anatomical physical barriers as well as physiological and cellular responses, which protect the host against harmful microorganisms while limiting host responses to harmless symbionts. Humans are home to 10 13 to 10 14 bacteria, roughly equivalent to the number of human cells, and while these bacteria can be pathogenic to their host most of them are mutually beneficial to both the host and bacteria.
The human immune system consists of two main types of immunity: innate and adaptive. The innate immune system, nonspecific defense mechanisms against foreign cells within the host, including the skin as a physical barrier to entry, activation of the complement cascade to identify foreign bacteria and to activate cell responses, and white blood cells that remove foreign matter. The adaptive immune system, acquired immunity, pathogen-specific immune response, which is carried out in lymphocytes by presentation of antigens on MHC molecules to distinguish between self and non-self antigens.
Microbes may contribute to the development of the immune system of the hosts in the intestine and skin, and can help to prevent pathogen invasion. Some produce anti-inflammatory agents, protection against parasitic intestinal microbes. Commensals contribute to the development of lymphocytes, which secrete protective antibodies, immunoglobulin a IgA At. This may neutralize pathogens and exotoxins and contribute to the development of immune cells and the immune response of the mucous membranes. However, the microbes have been implicated in human diseases, including inflammatory bowel disease, obesity and cancer.