1. Function
Forms the framework for production, maturation and circulation of immune cells.
Augments the venous circulation by draining the tissue fluid back to heart.
Control of interstitial tissue pressure
Important component of body defence mechanism-brings antigens in contact with immune cells.

2. Components
Lymph
Lymphatic vessels
Lymphoid tissues

3. Lymph Flows through the lymphatic system.
Passes at least once through lymph nodes before it joins the venous system.
Lymph is a clear colourless fluid.
Milky in colour in the intestinal vessels after digestion-is termed chyle.
Derived from the blood.
Consists of varieties of blood cells and sometimes microbes.

4. Lymphatic vessels Lymph vessels are:
- capillaries – smallest, starts blindly between cells, collects tissue fluids not absorbed by venous system.
- vessels-Formed by joining of the capillaries
- trunks-formed by joining of vessels in a region-Thoracic trunk and tracheal trunk.

5. One way pathway that parallels the venous system.
Movement (pushed) due to pressure gradient, muscle movements and valves.
Empties into cranial venacava or its branches
Vessels that carry lymph to lymph node is called afferent vessel
Vessel that carry lymph out of lymph node is called efferent vessel

7. Lymphoid tissues
Lymphoid tissues -Lymph nodes, nodules and diffused tissues, spleen, thymus and tonsils
Lymph Nodes:
small encapsulated organs located along the pathway of lymphatic vessels.
They vary from about 1 mm to 1 to 2 cm in diameter and are widely distributed throughout the body,
large concentrations occurring in the areas of convergence of lymph vessels.
serve as filters through which lymph percolates on its way to the blood.
Antigen-activated lymphocytes differentiate and proliferate by cloning in the lymph nodes. 

9. Circulation in lymph nodes

10. Lymph nodules and diffused tissues
found in  connective tissue beneath the epithelial mucosa.
The alimentary canal, respiratory passages, and genitourinary tract are guarded by accumulations of lymphatic tissue
Not enclosed by a capsule (i.e. they are diffuse
These cells intercept foreign antigens and then travel to lymph nodes to undergo differentiation and proliferation.

12. Local concentrations of lymphocytes in these systems and other areas are called lymphatic nodules.
In general these are single and random but are more concentrated in the GI tract in the ileum, appendix, cecum, and tonsils.
These are collectively called the Gut Associated Lymphatic Tissue (GALT). MALT (Mucosa Associated Lymphatic Tissue) includes these plus the diffuse lymph tissue in the respiratory tract. 

14.      3. The thymus 
   Located cranial to heart, with portions extending along the trachea.
 Organ of immature animals
when lymphocyte processing and proliferation are dramatically reduced, the thymic tissue is largely replaced by adipose tissue.
Immature lymphocytes differentiate into T-lymphocytes
T-lymphocytes survive for long periods and recirculate through lymphatic tissues.       

15. The transformation of primitive or immature lymphocytes into T-lymphocytes and their proliferation in the lymph nodes is promoted by a thymic hormone-thymosin. 
Ocassionally the thymus persists and may become cancerous -continued secretion of thymosin – continious T-cells may -autoimmune disorders. 
Thymosin has been used experimentally to stimulate T-lymphocyte proliferation to fight lymphoma and other cancers. 

16. The spleen Located attached to stomache
The spleen functions in both immune and hematopoietic systems
Immune functions- proliferation of lymphocytes, production of antibodies, removal of antigens from the blood.
Hematopoietic functions- formation of blood cells during fetal life, removal and destruction of aged, damaged and abnormal red cells and platelets, retrieval of iron from hemoglobin degradation, storage of red blood cells.

17. Tonsils Unencapsulated aggregate of lymphatic nodules
Associated with pharyngeal mucosa.
No afferent lymphatic vessels.
Make direct contact with the antigens through the mucus membrane.

18. Lymphatic circulation

19. Lymphatic system