It is the study of immune system response that occurs at the mucosal membrane of the intestines, the urogenital tracts, and respiratory systems, i.e. surfaces that are in contact with the external environment in healthy states, the mucosal immune system provides protection against pathogens but maintains a tolerance towards non-harmful commensal microbes and benign environmental substance. Since the mucosal membranes are the primary contact point between a host and its environment a large amount of secondary lymphoid tissue is found here. The mucosa-associated lymphoid tissues or MALT provide the organism with an important line of defense.