Abstract

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.

Author affiliations

1. Division of Biology, California Institute of Technology, Mail code 156-29, 1200 East California Boulevard, Pasadena, California 91125, USA.
2. Channing Laboratory, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, 181 Longwood Avenue, Boston, Massachusetts 02115, USA.

Correspondence to: Dennis L. Kasper² Email: dennis_kasper@hms.harvard.edu

Published online 6 October 2006

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