Bacteria and mucosal immunity

In normal individuals, the intestine is a site of intense immunological activity due to the continuous stimulation by luminal antigens mostly derived from the normal bacterial flora. This is reflected in the huge amount of IgA produced in the gut and the abundant T cells in the lamina propria and epithelium. It is also becoming clear that products of the normal flora may regulate the cytokine environment within the inductive sites of the mucosal immune responses, such as the Peyer's patches of the small bowel. Thus normal flora could either negatively or positively regulate specific immune responses by dictating the profile of locally released cytokines. For example, it is known that in Crohn's disease the antigens that drive the strongly polarized Th1 tissue-damaging response are derived from the normal bacteria flora. Emerging evidence also indicates that gut microflora can contribute to maintain the mucosal homeostasis by promoting the generation and/or expansion of counter-regulatory mechanisms.