• Provides immune surveillance and defense against foreign substances and microorganisms
• Provides immune tolerance, distinguishing between "self" and "non-self"
• Absorbs lipids into small lymphoid vessels (lacteals) in intestinal villi for distribution to the blood stream and liver
• Helps to maintain fluid balance by accumulating tissue fluid and white blood cells in lymph vessels and returning them to the blood
^ Overview of lymphoid components
• Primary lymphoid organs and structures
♦ Bone marrow. Site of origin of T and B lymphocytes. B lymphocytes directly seed secondary lymphoid structures and organs.
♦ Thymus. T lymphocytes from bone marrow undergo further maturation in the thymus before seeding secondary lymphoid structures and organs.
• Secondary lymphoid organs and structures (from least to most complex)
♦ Diffuse lymphoid tissue
♦ Lymphoid nodules. Both solitary and in aggregates.
• Major lymphoid cell types
♦ B lymphocytes originate and mature in the bone marrow, then seed secondary lymphoid structures and organs. B cells differentiate into B memory cells and plasma cells, providing humoral immunity.
♦ T lymphocytes originate in bone marrow, mature in the thymus, and subsequently seed secondary lymphoid tissue. T cells differentiate into helper, memory, and cytotoxic cells. T lymphocytes provide cell-mediated immunity and assist B lymphocytes in their humoral response.
♦ Plasma cells differentiate from B lymphocytes and produce humoral antibodies.
♦ Macrophages and dendritic cells phagocytose foreign matter, enhance the body's response to antigen by "presenting" antigen to lymphocytes, and secrete immunomodulatory factors.
• Lymph vessels
♦ Are thin-walled vessels lined with endothelium
♦ Begin as blind-ended lymphatic capillaries in tissues. They accumulate tissue fluid, which is called lymph once it is enclosed by the capillary.
♦ Gradually increase in diameter and have valves located within their walls. Lymph nodes are positioned along these vessels.
♦ Unite to form two lymph ducts (thoracic and lymphatic ducts) that return lymph to the venous side of the blood vasculature system
• High endothelial venules (HEVs)
♦ Located in appendix, tonsils, Peyer's patches, and especially in lymph nodes, but not in spleen
♦ Endothelium lining these venules is simple cuboidal rather than simple squamous epithelium
♦ Allows transport of lymphocytes through the endothelium, thus permitting diapedesis of these cells and the dissemination of immunological information between different regions of the body
• Stroma of lymphoid structures and organs
♦ Reticular cells produce reticular fibers and act as fixed macrophages as they ensheathe these fibers. This tissue constitutes reticular connective tissue.
♦ Reticular fibers are composed of collagen type III and form a meshwork that allows fluid to percolate through it while providing delicate, nondistensible support for cells suspended within it.
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