LYMPHOID TISSUES

The lymphatic system is involved with the clearing of infectious agents from the body. It transports the lymph which is the clear fluid originating from the plasma (liquid portion of blood). 

Functions

• Lymph drains interstitial fluid
• Transports dietary lipids 
•  Facilitates immune responses.

The plasma filters out of the capillaries and drains into lymphatic vessels to become lymph. It also transports fat. The lymph flows in the lymphatic vessels, lymphatic tissue and red bone marrow. The content of lymph is the same as interstitial fluid (the fluid around tissue cells). The Lymph eventually drains into venous blood via the right atrium.

What are the primary lymphatic organs? 

These are the sites where the lymphoid cells (T-LYMPHOCYTES and B-LYMPHOCYTES) are produced. It includes:

• Bone marrow
• Thymus

What are secondary lymphatic organs?

Secondary lymphoid tissues are arranged as a series of filters that monitor the contents of the extra-cellular fluids (lymph, interstitial fluid and blood). Secondary lymphoid tissues are also where lymphocytes are activated.

These include: 

• lymph nodes: The lymph is filtered by lymph nodes. These are encapsulated lymphoid tissue. There are about 100-200 lymph nodes, which mostly occur in the neck, thorax, abdomen and pelvis. They contain B-lymphocytes (cells) and T-lymphocytes (cells), which mostly enter the nodes via the blood stream. It also contains macrophages. 
• Mucosa Associated Lymphoid Tissue (MALT): These are non-encapsulated lymphoid aggregations found close to moist epithelial surfaces such as the mucous membranes of the digestive, respiratory and reproductive systems. They respond to any bacteria or micro-organisms that do get through the epithelia. The activated B-cells in these areas can develop into plasma cells, which then produce antibodies. They filter tissue fluid.  This makes up 85% of lymphoid tissue in the mucosa. They are usually small (around 1 millimeter in diameter), with the exception of the tonsils, peyers patches and the appendix.
• Tonsils are large partially-encapsulated masses of lymphoid tissue, found in the walls of the pharynx and nasopharynx, and at the base of the tongue. 
• Peyer's patches are large masses of confluent lymphoid follicles, found in the walls of the ileum (small intestine).
•  Spleen: The blood is filtered by the spleen, an encapsulated lymphoid tissue. This is the body's largest lymphatic organ. 

Functions of the spleen: 

• It is important for antibody production
• Facilitates immune responses to blood borne antigens 
•  It eliminates worn-out blood cells and platelets.