Mucosal surfaces constitute a large host-environmental interface that must be protected from pathogenic organisms. The mucosal immune system has evolved as a distinct immune organ functioning independently from its systemic counterpart. The mucosal immune system has the difficult task of mounting protective responses to invading microorganisms while maintaining a state of nonresponsiveness to commensal bacteria and food antigens. The system has unique cellular components and functional aspects that permit it to carry out this dual role.