<qa>  What is the mediator of mucosal protection?Prostaglandin E2 is one of the prostanoids, which is involved with innate immunity. PGE2 induces oral tolerance to specific antigen in the small intestine and downregulates the production and release of proinflammatory cytokines by macrophages and neutrophils. Accordingly, PGE2 is considered to be the mediator of mucosal protection. 