IgA and the Intestinal Microbiota: IgA is the dominant immunoglobulin isotype produced in mammals, largely secreted across the intestinal mucosal surface. IgA mediates microbial homeostasis at the intestinal mucosa. Within the gut, IgA acts in a context-dependent manner to both prevent and promote bacterial colonization and to influence bacterial gene expression, thus providing exquisite control of the microbiota.