Innate immunity of the gut: mucosal defense in health and disease.

The intestine is an important immune organ consisting of a complex cellular network, secreted peptides and proteins and other host defenses. Innate immunity plays a central role in intestinal immune defense against invading pathogens. It also serves as a bridge to the activation of the adaptive immune system. Pattern recognition molecules of microorganisms are an essential component for identifying invading pathogens. Toll-like receptors (TLRs), CARD15/NOD2 and scavenger receptors all serve as the pattern recognition receptors in the innate immune defense system. Secreted bactericidal peptides or defensins produced by the intestinal epithelia represent another crucial element of innate mucosal immune defense. Mutations in pattern recognition receptors and dysfunction of secretory bactericidal peptides may impair host immune defenses leading to an invasion of pathogens resulting in chronic inflammation of the gut. This review updates our current understanding of innate immunity of the gastrointestinal tract.