New aspects of IgA synthesis in the gut

In mammals, the gastrointestinal tract is colonized by extremely dense and diverse bacterial communities that are beneficial for health. Maintenance of the complexity and the proper localization and distribution of gut bacteria is of prime importance because when disrupted, the microbial community attacks the host's tissues and causes inflammatory reactions. Our immune system provides the necessary mechanisms to maintain the homeostatic balance between microbial communities and the host. IgA plays crucial roles in regulation of host-bacteria interactions in the gut. IgA is the most abundant immunoglobulin isotype in our body, mostly produced by the IgA plasma cells residing in the lamina propria of the small and large intestine. Although it was well known that IgA provides protection against pathogens, only recently has it become clear that IgA plays critical roles in regulation of bacterial communities in the gut in steady-state conditions. Here, we summarize recent progress in our understanding of the various mechanisms of IgA synthesis in multiple anatomical sites and discuss how IgA limits bacterial access to the internal milieu of the host.