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Understanding Cancer: The Immune System

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The Immune System

The Immune System
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An Overview of the Specific Immune System

What is the Specific Immune System?

The specific immune system is your body's way of defending you against pathogens, cells that cause disease or harm, viruses, and even your own cells (if they are cancerous).  The main group of cells that functions in the specific immune system is white blood cells, or leukocytes.  White blood cells are cells that live in the bloodstream and move around to fight infections via blood vessels; they defend the human body against bacterial and viral infections, fungal infections, and any invasive or harmful cells.  White blood cells do this through several different ways: some secrete chemicals that kill pathogens (disease-giving cells), others may induce, or cause, pathogenic cells to undergo apoptosis, which is the process in which a cell commands its own components to turn on it and commit suicide (programmed cell death), while some leukocytes may simply inactivate viruses and other intruders; still other leukocytes (white blood cells) may "sound" the alarm by releasing molecules that activate other white blood cells.  White blood cells can be further divided into various categories, but for the purpose of our discussion, we will only consider lymphocytes in-depth.  There are two types of lymphocytes: B lymphocytes, which are usually called B cells, and T lymphocytes, which are usually called T cells. 

B Cells

B cells are produced in the bone marrow and mature there.  B cells possess protein receptors on the surface of their cell walls; these receptors are each specific to certain pathogens or protein complexes on the surface of other cells.  If a B cell encounters the pathogen that the B cell is specific to, then the B cell binds to the pathogen at the epitope and engulfs the pathogen.  The B cell then uses its lysosomes to cut the pathogen up and it sends a piece of the pathogen to its MHC (Major Histocompatibility Complex) II, a protein complex that allows the B cell to “present” a piece of a pathogen to other body cells.  In most cases, the B cell does not activate just because it has engulfed a foreign cell.  The B cell waits for the arrival of an activated helper T cell that is specific to the same pathogen that the B cell is.  If activated, the B cell begins to proliferate rapidly; two types of B cells are formed: effector B cells and memory B cells.  Generally, effector B cells, which are more correctly called “plasma cells,” begin to produce massive amounts of antibodies to combat the pathogen.  These antibodies bind to the pathogen and either tag it for killing, or inactivate it.  The memory B cells, on the other hand, stay in the bloodstream for a long time afterwards, and keep the immune system prepared for the possibility of a future attack by the same kind of pathogen.

T Cells

Are produced in the bone marrow, but mature in the thymus.  There are two main divisions for T cells: cytotoxic T cells and helper T cells.  Helper T cells are activated by coming in contact with a cell that has consumed a pathogen that the T cell is specific to.  Dendritic cells apparently are best at activating helper T cells.  Once activated, the helper T cells begin dividing rapidly, forming effector and memory cells.  Effector helper T cells release large quantities of cytokines, which are signal proteins or polypeptides, into the bloodstream.  The cytokines prepare the immune system for defense.  The effector helper T cells then move through the bloodstream until they come in contact with a B cell that has the same specificity as the helper T cell.  The two cells bind at the B cell’s MHC II complex and the B cell becomes activated.  Cytotoxic T cells, on the other hand, bind to MHC I complexes on the outer surface of cells.  MHC I complexes are on the surface of all body cells that have a nucleus, which means only red blood cells do not have them.  The MHC I complex is used by the cell to “present” inner components of the cell to outside cells, such as cytotoxic T cells.  If a cell is experiencing a genetic abnormality, such as cancer, or is infected by a virus, the cell sends evidence to the MHC I complex.  When a cytotoxic T cell comes in contact with a cell, it binds to its MHC I complex; if the cell is cancerous or infected, then the cytotoxic T cell becomes activated.  Upon activation, the cytotoxic T cell proliferates into effector and memory cells.  Effector cells immediately begin to kill or induce the death of infected or cancerous cells.  Sometimes, an effector cytotoxic T cell will release proteins called poriferins.  These poriferins penetrate the infected cell, producing holes in the cell’s plasma membrane which allow a set of proteins known as granzymes to enter.  These granzymes bring about the death of the cell.  Other times, an effector cell will stimulate a cell to undergo apoptosis, programmed cell death.  Either way, cytotoxic T cells usefully eliminate diseased cells.  Memory cytotoxic T cells stay around for a while afterwards in case of the future development of cancerous or pathogenic cells that they are specific to attacking the immune system again.  T cells that bind to MHC I complexes are generally known as CD8+ T cells, while T cells that bind to MHC II complexes are called CD4+ T cells.