The love–hate relationship between bacterial polysaccharides and the host immune system

The commensal gut bacteriumBacteroides fragilismight be the key to a healthy immune system. A zwitterionic polysaccharide that is produced by this bacterium has immunomodulatory properties, and the authors suggest that this supports the 'hygiene hypothesis' at a molecular level. This article explores the fascinating relationship between the mammalian immune system and the bacteria that are present in the mammalian gut. Every human is an ecosystem that hosts 1013–1014 bacteria. We review the evidence that immunomodulatory molecules produced by commensal bacteria in the gut have a beneficial influence on the development of certain immune responses, through eliciting the clonal expansion of CD4+ T-cell populations. This process seems to contribute to the overall health of the host by offering protection against various diseases and might provide supporting evidence at a molecular level for the 'hygiene hypothesis' of allergic immune disorders.