Lymphoid system
Function:1- Provides immune surveillance and defense against foreign substances and
microorganisms.
2- Provides immune tolerance, distinguishing between "self" and "non-self"
3- Absorbs lipids into small lymphoid vessels (lacteals) in intestinal villi for distribution
to the blood stream and liver.
4- 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
1. Primary Lymphoid organs and structures
- Bone marrow. Site of origin of T and B lymphocytes.
- Thymus T lymphocytes from bone marrow undergo further maturation in the thymus
before seeding secondary lymphoid structure and organs.
2. Secondary lymphoid organs and structures (from least to most complex)
- Diffuse lymphoid tissue
- Lymphoid nodules Both solitary and in aggregates.
- Tonsils
- Lymph nodes
- Spleen
3- Major lymphoid cell type
- B lymphocytes originate and mature in the bone marrow, then seed secondary
lymphoid structures and organs.
- T lymphocytes orginate in bone marrow, mature in the thymus, and subsequently
seed secondary lymphoid tissue.
- 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.
4- Lymph vessels
Are thin-walled vessels lined with endothelium, Gradually increase in diameter and have
valves located within their walls. Lymph nodes are positioned along these vessels.
5- High endothelial venules (HEVs)
- Located in appendix, tonsils, Peyer's patches, and especially in lymph nodes. But not
in spleen
- Endothelium lining these venules in simple cuboidal rather than simple squamous
epithelium
6- Stroma of lymphoid structures and organs
- Reticular cells produce reticular fibers and act as fixed macrophages as they
ensneathe 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.
Components of the lymphoid system
1- Diffuse lymphatic tissue
Located in lamina propria of any organ system opening to the exterior of the body such as
respiratory and digestive systems, where an antigen could penetrate the epithelium and
enter the lamina propria. Composed og an unorganized cluster of lymphocytes and other
cells capable of responding to an antigen that reaches it.
2- Lymphoid nodules
Mary occur singly (solitary) or in clusters (aggregates) such as in tonsils and Peyer's patches
in the small intestine. It consists of:
- Primary nodule. The nodule present before antigen stimulation, it consists primarily
of densely packed spheres of B lymphocytes.