Peyer's patches, M cells, lamina propria lymphocytes and secretory IgA
Expansion
The gut has the largest lymphoid mass in the body, facing an enormous antigen load while needing to remain tolerant to food and commensals.
Components:
- Peyer’s patches: aggregated lymphoid follicles, most numerous in the ileum
- M cells: specialised epithelial cells overlying the follicles that sample luminal antigen and pass it to underlying immune cells
- Lamina propria: plasma cells, T cells, macrophages and dendritic cells
- Intraepithelial lymphocytes, mostly CD8 T cells
- Mesenteric lymph nodes
Secretory IgA is the principal effector: made as a dimer, it acquires a secretory component as it is transported across the epithelium, which resists proteolysis. It works by immune exclusion, binding organisms and toxins without provoking inflammation.
Oral tolerance is the active suppression of responses to harmless antigens; its failure underlies coeliac disease and food allergy.
M cells are also the route of entry exploited by Salmonella, Shigella, Yersinia and poliovirus, and increased intraepithelial lymphocytes are the earliest histological sign of coeliac disease.