M cells endocytose antigens from the surface of the mucosa and exocytose them to the bodies interior where macrophages and dendritic cells reside. If coated with antibodies or complement, the antigens will be very effectively captured by dendritic cells for display to B cells in nearby follicles. These organized centers collect antigen from macrophages that can migrate from the mucocal area. Intraepithelial lymphocytes are present throughout the mucosa. Their function is unclear.
Tonsils are nodular organs in the upper respiratory system.
They contain several lymphoid follicles. Skin contains intraepithelial lymphocytes whose function is still somewhat unclear. Langerhans cells are dendritic cells in skin that migrate via blood and lymphatic vessels to lymph nodes when they have antigens to present. See figure 2-23 . Of particular interest is the immune system associated with the gastrointestinal tract.