Secretory IgA (SIgA) serves as the first line of defense in protecting the intestinal epithelium from enteric toxins and pathogenic microorganisms. Through a process known as immune exclusion, SIgA promotes the clearance of antigens and pathogenic microorganisms from the intestinal lumen by blocking their access to epithelial receptors, entrapping them in mucus, and facilitating their removal by peristaltic and mucociliary activities. In addition, SIgA functions in mucosal immunity and intestinal homeostasis through mechanisms that have only recently been revealed. In just the past several years, SIgA has been identified as having the capacity to directly quench bacterial virulence factors, influence the composition of the intestinal microbiota by Fab-dependent and -independent mechanisms, promote the retro-transport of antigens across the intestinal epithelium to dendritic cell (DC) subsets in gut-associated lymphoid tissue, and, finally, to down-regulate pro-inflammatory responses normally associated with the uptake of highly pathogenic bacteria and potentially allergenic antigens. This review summarizes the intrinsic biological activities now associated with SIgA and their relationships to immunity and intestinal homeostasis.

Background:Intestinal commensal bacteria are not ignored by the mucosal immune system, yet the mechanisms ensuring homeostatic communication are poorly defined. Results: Coating of commensals by SIgA mediates their targeting to Peyer's patch dendritic cells. Conclusion: SIgA is important for the natural dynamic host-microbiota dialogue. Significance: Beyond pathogens, immune surveillance function of SIgA applies to the control of commensal bacteria.

Material Supplementary 5.DC1http://www.jimmunol.org/content/suppl/2010/03/26/jimmunol.090169References

Antigenemia in patients with Mediterranean visceral leishmaniasis (MVL) due to Leishmania infantum was retrospectively assessed by sandwich enzyme-linked immunosorbent assay (ELISA). Circulating Leishmania antigens, partially in free form, were in evidence in 53% of serum samples from immunocompetent individuals with MVL. Following successful therapy, antigenemia decline as measured by ELISA was more pronounced than antibody decrease.