Question

Discuss mucosal immunity, in terms of its physical organization as well as describing how it

actually functions.

Answer 1

The immune system may be viewed as an organ that is distributed throughout the body to provide host defense against pathogens wherever these may enter or spread. Within the immune system, a series of anatomically distinct compartments can be distinguished, each of which is specially adapted to generate a response to pathogens present in a particular set of body tissues. We also have an adaptive immune response in the compartment comprising the peripheral lymph nodes and spleen. This is the compartment that responds to antigens that have entered the tissues or spread into the blood. A second compartment of the adaptive immune system of equal size to this, and located near the surfaces where most pathogens invade, is the mucosal immune system (commonly described by the acronym MALT). Two further distinct compartments are those of the body cavities (peritoneum and pleura) and the skin. Two key features define these compartments. The first is that immune responses induced within one compartment are largely confined in expression to that particular compartment. The second is that lymphocytes are restricted to particular compartments by expression of homing receptors that are bound by ligands, known as addressins, that are specifically expressed within the tissues of the compartment. We will illustrate the concept of compartmentalization of the immune system by considering the mucosal immune system. The mucosal surfaces of the body are particularly vulnerable to infection. They are thin and permeable barriers to the interior of the body because of their physiological activities in gas exchange (the lungs), food absorption (the gut), sensory activities (eyes, nose, mouth, and throat), and reproduction (uterus and vagina). The necessity for permeability of the surface lining these sites creates obvious vulnerability to infection and it is not surprising that the vast majority of infectious agents invade the human body through these routes.

A second important point to bear in mind when considering the immunobiology of mucosal surfaces is that the gut acts as a portal of entry to a vast array of foreign antigens in the form of food. The immune system has evolved mechanisms to avoid a vigorous immune response to food antigens on the one hand and, on the other, to detect and kill pathogenic organisms gaining entry through the gut. To complicate matters further, most of the gut is heavily colonized by approximately 1014 commensal microorganisms, which live in symbiosis with their host. These bacteria are beneficial to their host in many ways. They provide protection against pathogenic bacteria by occupying the ecological niches for bacteria in the gut. They also serve a nutritional role in their host by synthesizing vitamin K and some of the components of the vitamin B complex.

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.