Regional Lymphoid Tissue - Anatomy & Physiology

Introduction

Mucosal associated lymphoid tissue (MALT) covers lymphoid tissues associated with the mucosal surfaces of alimentary, respiratory, urinary and reproductive tracts. Due to the extent of these surfaces the mucosal lymphoid tissue contains as many lymphocytes as the rest of body. The MALT is strategically located to intercept pathogens before they enter the body.

Development

γδ T cells migrate to these tissues during foetal development from the thymus.

Structure

The MALT is found in lamina propria and is non-encapsulated lymphoid tissue. In the alimentary and respiratory tract it is more specifically called:

Gut associated lymphoid tissue (GALT)
- Including the Peyer’s patches, tonsils, appendix, and lymphoid follicles in large intestine and rectum
Bronchial associated lymphoid tissue (BALT)
- Located at the bronchial bifurcations under non-ciliated epithelium

Lymphatic nodules are present in the mucosal surfaces and are more readily defined than other more diffuse lymphoid tissue of the MALT but are still not encapsulated. They exist with small primary nodule or generally as secondary nodules containing germinal centres. The nodules are generally found in the tonsils, Peyer’s patches and appendix and become enlarged when responding to an antigen.

Functions

As with other secondary lymphoid tissue the role of the MALT is to mount an immune response to pathogenic antigens. Cells in the MALT sample antigens at the mucosal surface and then migrate to regional lymph nodes where they divide and differentiate before returning to the mucosal surface as effector cells.

In the GALT TGF-β is produced and this causes the B cells to produce IgA.