The GALT comprises the Peyer’s patches, which are present only in the small intestine; isolated lymphoid follicles (ILF), which are found throughout the intestine; the appendix (in humans); and the tonsils and adenoids in the throat. The palatine tonsils, adenoids, and lingual tonsils are large aggregates of lymphoid tissue covered by a layer of squamous epithelium and form a ring, known as Waldeyer’s ring, at the back of the mouth around the entrance of the gut and airways (Fig. 12.6). They often become extremely enlarged in childhood because of recurrent infections, and in the past were frequently removed as a result. A reduced IgA response to oral polio vaccination has been seen in individuals who have had their tonsils and adenoids removed.
The tonsils and adenoids form a ring of lymphoid tissues, Waldeyer’s ring, around the entrance of the gut and airway adenoid palatine tonsil lingual tonsil tongue
Fig. 12.6 A ring of lymphoid organs called Waldeyer’s ring surrounds the entrance to the intestine and respiratory tract. The adenoids lie at either side of the base of the nose, while the palatine tonsils lie at either side of the back of the oral cavity. The lingual tonsils are discrete lymphoid organs on the base of the tongue. The micrograph shows a section through an inflamed human tonsil, where the areas of organized lymphoid tissue are covered by a layer of squamous epithelium (at top of photo). The surface contains deep crevices (crypts) that increase the surface area but can easily become sites of infection. Hematoxylin and eosin staining. Magnification ×100.
Fig. 12.7 Transport of antigens by M cells facilitates antigen presentation.
The first panel illustrates the passage of antigen through M cells in the follicle‑associated epithelium of Peyer’s patches. M cells have convoluted basal membranes that form ‘pockets’ within the epithelial layer, allowing close contact with lymphocytes and other cells. This favors the local transport of antigens that have been taken up from the intestine by the M cells and their delivery to dendritic cells for antigen presentation. The top right micrograph of a Peyer’s patch stained with fluorescently labeled antibodies shows epithelial cells (cytokeratin, dark blue), with M‑cell pockets inferred from the presence of T cells (CD3, red) and B cells (CD20, green). At bottom right, Peyer’s patch follicle epithelium shows CX3CR1expressing myeloid cells (green), which include some dendritic cells, interacting with M cells identified by expression of peptidoglycan recognition protein‑S (red) and apical staining for the UEA‑1 lectin (cyan). Some CX3CR1‑expressing cells extend processes into the M cells (arrows). Top right, micrograph from Espen, S., et al.: Immunol. Today 1999, 20:141–151. Bottom right, micrograph from Wang et al.: J. Immunol. 2011, 187:5277–5285.