The mucosa of the digestive, respiratory and urinary tracts often contains small aggregations of lymphocytes called lymphoid follicles. These are called 'Mucosa associated lymphoid tissue' (MALT).

In some cases, these aggregations are large, and confluent. This happens in the tonsils, peyers patches and the appendix.

Peyers patches mostly contain T-cells, but also can have germinal centres that contain B-lymphocytes, as well as macrophages.

Peyer's patches to not have any afferent lymphatics. The activated lymphocytes pass out in efferent lymphatics and travel to the lymph nodes.

This shows a diagramatic representation of an M-cell, and its function.

The epithelium of the mucosa contains special flat epithelial cells called M (membrane-like) cells or FAE (follicle associated epithelial cells). These specialised cells take up small amounts of gut antigens entering the gut lumen, and pass them onto antigen presenting cells, and lymphocytes of the MALT. Diffuse B-lymphocytes in gut react to micro-organisms that cross the mucosa, and they make IgA type antibodies, which are then secreted directly onto the gut lumen. These antibodies help to prevent micro-organisms in the gut from sticking to the gut epithelium, and can neutralise toxins and viruses.