Lymphatic System

Functions of the Lymphatic System:

1. Fluid Balance
2. Fat absorption
3. Defense

Overview of the Lymphatic System:

1. Lymphatic capillaries remove fluid from tissues.  The fluid becomes lymph.
2. Lymph flows through lymphatic vessels, which have valves that prevent the back flow of lymph.
3. Lymph nodes filter lymph and are sites where lymphocytes respond to infections.
4. Lymph enters the thoracic duct or the right lymphatic duct.
5. Lymph enters the blood.
6. Lacteals in the small intestine absorb fats, which enter the thoracic duct.
7. Chyle, which is lymph containing fats, enters the blood.
8. The spleen filters blood and is a site where lymphocytes respond to infections.
9. Lymphocytes (pre-B and pre-T cells) originate from stem cells in the red bone marrow.  The pre-B cells become mature B cells in the red bone marrow and are released into the blood. The pre-T cells enter the blood and migrate to the thymus.
10. The thymus is where pre-T cells derived from red bone marrow increase in number and become mature T cells that are released into the blood.
11. B cells and T cells from the blood enter and populate all lymphatic tissues.  These lymphocytes can remain in tissues or pass through them and return to the blood.  B cells and T cells can also respond to infections by dividing and increasing in number.  Some of the newly formed cells enter the blood and circulate to other tissues.

Proliferation of Helper T Cells:

1. Antigen-presenting cells, such as macrophages, phagocytize, process, and display antigens on the cell's surface.
2. The antigens are bound to MHC class II molecules, which present the processed antigen to the T-cell receptor of the helper T cell.
3. Costimulation results from interleukin-1, secrete by the macrophage, and the CD4 glycoprotein of the helper T cell.
4. Interleukin-1 stimulates the helper T cell to secrete interleukin-2 and to produce interleukin-2 receptors.
5. The helper T cell stimulates itself to divide when interleukin-2 binds to interleukin-2 receptors.
6. The "daughter" helper T cells resulting from this division can be stimulated to divide again if they are exposed to the same antigen that stimulated the "parent" helper T cell. This greatly increases the number of helper T cells.
7. The increased number of helper T cells can facilitate the activation of B cells or effector T cells.

Classes of Antibodies and their functions

Proliferation of Cytotoxic T Cells:

1. An MHC class I molecule displays an antigen, such as viral protein, on the surface of a target cell.
2. The activation of cytotoxic T cell begins when the T-cell receptor binds to the MHC class I/antigen complex.
3. There is costimulation of the cytotoxic T cell by CD8 and other surface molecules.
4. There is costimulation by cytokines, such as interleukin-2, released from helper T cells.
5. The activated cytotoxic T cell divides, the resulting daughter cell divide, and so on, eventually pro ducting many cytotoxic T cells.