Plate 10.203 Ileum: Lymphocytic aggregation
Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger,
Peer Review Status: Externally Peer Reviewed
Human, 10% formalin, H. & E., 96 x.
Lymphocytes leave lymphoid organs to populate Peyer's patches of the ileum and elsewhere. B lymphocytes that arise from the bone marrow are found in Peyer's patches. When activated, they give rise to lymphoblasts, which differentiate into plasma cells and small lymphocytes called memory cells. Plasma cells migrate through the lamina propria to the basal lamina of the epithelial surface where they secrete IgA (see section on lymphatic system), which is transported through the overlying epithelium to provide antibody at the epithelial surface of the organ. Other immunoglobulin (IgM, IgG)- secreting cells are also present but are fewer in number.
The epithelium overlying the massive aggregation of (primarily) B lymphocytes is squamous to low columnar. The epithelial cells are called M cells and are thought to endocytose antigens from the lumen of the organ, transport them through their cytoplasm, and discharge them so that they may make contact with the active, underlying lymphocytes.
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