Mucosal immune system: A brief review

Most of antigens that encounter the Immune System along life enter the body through mucosal surfaces of the respiratory, gastrointestinal and urogenital tract. These are the largest areas within the body in contact with the external environment and in adult humans protect some 400 m2 of surface. Mucosal surfaces separate the external environment from the internal sterile environment and so represent a first line of defence system. This barrier faces environments rich in pathogens, which have developed effective mechanisms for colonisation of epithelial surfaces and invasion of mucosal tissues, but also harmless antigens such as food, airborne antigens or commensal bacterial flora. The latter represent the vast majority of the encountered antigens and require an appropriate response characterised by either ignorance or active suppression. However, for the former, a robust immune response is needed. Under these influences, mucosae have developed a complex immune system, anatomical and functionally different from the systemic immune system, which is capable of mounting an immune response against pathogenic antigens while maintaining the required ignorance or active suppression against non-pathogenic antigens.