MUCOSAL IMMUNITY

The mucosal immune system is distinct from its systemic counterpart. The peripheral immune system is characterized primarily by its ability to eradicate foreign antigens and functions in a relatively antigen-free environment. In contrast, the mucosal immune system is in constant juxtaposition with luminal flora and dietary proteins. Therefore, instead of mounting active immune responses, which would result in devastating consequences to the host, the general immune response of the gut is suppression. These responses are supported by several phenomena that have been observed in the gut, including oral tolerance and controlled or physiological inflammation. Suppression appears to be a selective process, as it does not preclude the ability of the gut to mount appropriate responses to pathogens (e.g., secretory IgA response). This emphasizes the dynamic ability of the mucosal immune system to adapt to environmental stimuli in a way that best suits the needs of the host. Aberrations in this balance result in inflammatory diseases of the bowel and food allergy.

The alternative pathways of immune regulation observed in the mucosal immune system are most likely explained by the distinct organization of the lymphoid structures and lymphocyte populations that are present. At this point, study of the anatomy and components of the mucosal immune system will facilitate understanding of the mechanisms involved in both health and disease of the gastrointestinal (GI) tract.

Anatomy of the Gut-Associated Lymphoid Tissue

Anatomically, the gut-associated lymphoid tissue (GALT) is composed of a single layer of epithelial cells separating the external environment from the underlying loose connective tissue, the lamina propria (LP) (Figure 14.1).