Microbial symbiosis and immunity

This article needs attention from an expert in Molecular and Cell Biology or Microbiology. The specific problem is: content unfocused, poor structure, bulk written by inexperienced student editor. WikiProject Molecular and Cell Biology or WikiProject Microbiology may be able to help recruit an expert. (February 2017)

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.^[1] 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 10¹³ to 10¹⁴ bacteria, roughly equivalent to the number of human cells,^[2] 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 is made of non-specific defensive mechanisms against foreign cells inside the host including skin as a physical barrier to entry, activation of the complement cascade to identify foreign bacteria and activate necessary cell responses, and white blood cells that remove foreign substances. The adaptive immune system, or acquired immune system, is a pathogen-specific immune response that is carried out by lymphocytes through antigen presentation on MHC molecules to distinguish between self and non-self antigens.

Microbes can promote the development of the host's immune system in the gut and skin, and may help to prevent pathogens from invading. Some release anti-inflammatory products, protecting against parasitic gut microbes. Commensals promote the development of B cells that produce a protective antibody, Immunoglobulin A (IgA). This can neutralize pathogens and exotoxins, and promote the development of immune cells and mucosal immune response. However, microbes have been implicated in human diseases including inflammatory bowel disease, obesity, and cancer.