Thymus, lymph node, spleen, accessory lymphoid tissue, mononuclear phagocyte system, lymphocyte ultrastructure

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Microbio, Immun, Pathology
MIP 343
Alan Schenkel

29 January Thymus Located in the anterior mediastinum left of the heart Central lymphoid organ Site of T cell maturation Lobules Pale medulla in center – consists of small lymphocytes and epithelial cells Smaller pink zones – Hassal’s corpuscles Darker cortex – consists of immature T cells which undergo maturation upon migration into the medulla, from which they enter the blood Changes in structure during maturation and aging in humans Reaches maximal size in adolescence Cortex progressively replaced by fat Similar changes occur in all mammals Lymph node Lymph containing macrophages drains through the afferent lymphatic vessel into the subcapsular sinus, through the intermediate sinuses, into the medullary sinuses, and out through the efferent lymphatic vessel. Lymph follicles, containing germinal centers and a small lymphocyte mantle, are sites of B lymphocytes proliferation. T-lymphocytes localize primarily in the paracortical zone. Medullary cords contain plasma cells. When stimulated Large number of lymph follicles with germinal centers Paracortical zone between lymphoid follicles Medulla less cellular because of medullary sinuses T lymphocytes – paracortical zone B lymphocytes – lymphoid follicles Some T cells in germinal centers to help B cells, some B cells in medullary cords differentiating into plasma cells Some B cells contain pink-staining immunoglobulin protein (Russell bodies) Germinal centers Contain pale zone consisting of dendritic cells, accumulate antigen on their surface to present to lymphocytes Darker, basophilic zone consisting of proliferating B lymphocytes Surrounded by thin small lymphocyte mantle Site of antigen focusing and proliferation of immunoresponsive cells If senescent: thicker small lymphocyte mantle (these lymphocytes did not originate within the germinal center, but simply have accumulated around it) Plasma cells, ultrastructurally, contain extensive rough endoplasmic reticulum dilated with immunoglobulin protein. Spleen Trabecular artery enters splenic pulp to form a central artery. This vessel is surrounded
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