The functional thymus consists of two cell populations; the stromal cells and the thymocytes. The stromal cells include (1) the subcapsular epithelial cells; (2) the cortical epithelial cells; (3) the medullary epithelial cells; (4) macrophages; and (5) dendritic cells of bone marrow origin. (From Kierszenbaum AL: Histology and cell biology: an introduction to pathology, St Louis, 2002, Mosby.)
BURSA OF FABRICIUS (B cell maturation) follicle lumen Cortex (lymphocytes, M Ø, plasma cells) medulla
PEYER’S PATCHES Group I (90% Ileum ) Ruminants, Pigs, Horses, Dogs, Humans. Group II Illeum/Jejunum) Rabbits, Rodents.
When stimulated by antigen, immunoglobulin A (IgA)-producing B cells are produced in inductive sites, such as the Peyer’s patches.
The role of M cells as antigen-processing cells in the intestinal wall. Antigen that enters enterocytes is usually rapidly degraded in lysosomes. Antigen that enters M cells is not degraded. It may be presented directly to intraepithelial lymphocytes within the M. Yersinia, M. tuberculosis, Listeria, reovirus , Salmonella.
Structure of the two different types of Peyer’s patch (PP) in sheep. A, An ileal PP at age 8 weeks. B, A PP from the jejunum, also at 8 weeks. Original magnification ×32. (From Reynolds JD, Morris B: Eur J Immunol 13:631, 1983.)