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. The previous part of the chapter illustrated the general principles underlying the initiation of 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.

The mucosa-associated lymphoid tissues lining the gut are known as gut-associated lymphoid tissue or GALT. The tonsils and adenoids form a ring, known as Waldeyer's ring, at the back of the mouth at the entrance of the gut and airways. They represent large aggregates of mucosal lymphoid tissue, which often become extremely enlarged in childhood because of recurrent infections, and which in the past were victims of a vogue for surgical removal. A reduced IgA response to oral polio vaccination has been seen in individuals who have had their tonsils and adenoids removed, which illustrates the importance of this subcompartment of the mucosal immune system.

In addition to the organized lymphoid tissue in which induction of immune responses occurs within the mucosal immune system, small foci of lymphocytes and plasma cells are scattered widely throughout the lamina propria of the gut wall. These represent the effector cells of the gut mucosal immune system. The life history of these cells is as follows. As naive lymphocytes, they emerge from the primary lymphoid organs of bone marrow and thymus to enter the inductive lymphoid tissue of the mucosal immune system via the bloodstream.

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Microbiology: Current Research
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