How does an individual mammal distinguish its own cells and proteins from those of its invading enemies?

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How does an individual mammal distinguish its own cells and proteins from those of its invading enemies?

When bacteria or viruses get through our body’s outer defences they may cause disease. To prevent this we have an immune system made up of cells and molecules that are constantly on patrol looking for and destroying foreign organisms.  For the system to be effective the cells and molecules that are involved must be able to recognise foreign material and distinguish it from similar chemicals that make up our tissues. This is a remarkable feat of molecular recognition (Alberts et al 1983 p.951-952).

Any material that can trigger the immune system into action is called an antigen. Antigens are foreign macromolecules not usually present in the body; they are the proteins, polysaccharides or glycoproteins that are on the surface of invading bacteria or viruses. Some of these invading pathogens produce harmful chemicals, called toxins. Toxins may also be antigens (http://www.bhs.berkeley.k12.ca.us/departments/Science/anatomy/anatomy97/immune/html/antibodygen.html).

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White blood cells (lymphocytes) play an important part in the immune response.  Production of lymphocytes starts in the bone marrow.  Once mature, the cells circulate through the blood and lymphatic vessels.  Lymphocytes fall into two groups—the B cells and the T cells (Alberts et al 1983 p.952).

B cells provide what is called humoral immunity.  The B cells have glycoproteins, called immunoglobulins or antibodies, displayed on their surface.  Each B cell displays many copies of just one antibody, which recognises a particular antigen and binds to it.  The binding of an antibody to an antigen must happen first in order ...

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