Antibody and antigen

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Photo by: cglightNing

Antibodies, also called immunoglobulins, are proteins manufactured by the body that help fight against foreign substances called antigens. When an antigen enters the body, it stimulates the immune system to produce antibodies. (The immune system is the body's natural defense system.) The antibodies attach, or bind, themselves to the antigen and inactivate it.

Every healthy adult's body has small amounts of thousands of different antibodies. Each one is highly specialized to recognize just one kind of foreign substance. Antibody molecules are typically Y-shaped, with a binding site on each arm of the Y. The binding sites of each antibody, in turn, have a specific shape. Only antigens that match this shape will fit into them. The role of antibodies is to bind with antigens and inactivate them so that other bodily processes can take over, destroy, and remove the foreign substances from the body.

Antigens are any substance that stimulates the immune system to produce antibodies. Antigens can be bacteria, viruses, or fungi that cause infection and disease. They can also be substances, called allergens, that bring on an allergic reaction. Common allergens include dust, pollen, animal dander, bee stings, or certain foods. Blood transfusions containing antigens incompatible with those in the body's own blood will stimulate the production of antibodies, which can cause serious, potentially life-threatening reactions.

Classes of antibodies and their functions

There are five classes of antibodies, each having a different function. They are IgG, IgA, IgM, IgD, and IgE. Ig is the abbreviation for immunoglobulin, or antibody.

IgG antibodies are the most common and the most important. They circulate in the blood and other body fluids, defending against invading bacteria and viruses. The binding of IgG antibodies with bacterial or viral antigens activates other immune cells that engulf and destroy the antigens. The smallest of the antibodies, IgG moves easily across cell membranes. In humans, this mobility allows the IgG in a pregnant woman to pass through the placenta to her fetus, providing a temporary defense to her unborn child.

IgA antibodies are present in tears, saliva, and mucus, as well as in secretions of the respiratory, reproductive, digestive, and urinary tracts. IgA functions to neutralize bacteria and viruses and prevent them from entering the body or reaching the internal organs.

IgM is present in the blood and is the largest of the antibodies, combining five Y-shaped units. It functions similarly to IgG in defending against antigens but cannot cross membranes because of its size. IgM is the main antibody produced in an initial attack by a specific bacterial or viral antigen, while IgG is usually produced in later infections caused by the same agent.

IgD is present in small amounts in the blood. This class of antibodies is found mostly on the surface of B cells—cells that produce and release antibodies. IgD assists B cells in recognizing specific antigens.

IgE antibodies are present in tiny amounts in serum (the watery part of body fluids) and are responsible for allergic reactions. IgE can bind to the surface of certain cells called mast cells, which contain strong chemicals, including histamine. (Histamines are substances released during an allergic reaction. They cause capillaries to dilate, muscles to contract, and gastric juices to be secreted.) When an allergen such as pollen binds with its specific IgE antibody, it stimulates the release of histamine from the mast cell. The irritating histamine causes the symptoms of an allergic reaction, such as runny nose, sneezing, and swollen tissues.

Tests that detect the presence of specific antibodies in the blood can be used to diagnose certain diseases. Antibodies are present whenever antigens provoke an immune reaction in the test serum.

The immune response

When a foreign substance enters the body for the first time, symptoms of disease may appear while the immune system is making antibodies to fight it. Subsequent attacks by the same antigen stimulate the immune memory to immediately produce large amounts of the antibody originally created. Because of this rapid response, there may be no symptoms of disease, and a person may not even be aware of exposure to the antigen. They have developed an immunity to it. This explains how people usually avoid getting certain diseases—such as chicken pox—more than once.

Immunization

Immunization is the process of making a person immune to a disease by inoculating them against it. Inoculation is the introduction of an antigen into the body—usually through an injection—to stimulate the production of antibodies.

The medical practice of immunization began at the end of the eighteenth century, when English physician Edward Jenner (1749–1823) successfully used extracts of body fluid from a dairymaid (a woman employed in a dairy) infected with cowpox (a mild disease) to inoculate a young boy against smallpox, a then-common and often fatal viral disease. Jenner called his method "vaccination," using the Latin words vacca, meaning "cow," and vaccinia, meaning "cowpox." Because the two diseases are caused by similar viruses that have the same antigens, antibodies that work against cowpox will also fight smallpox.

In 1885, a rabies vaccine developed by French scientist Louis Pasteur (1822–1895) from the spinal fluid of infected rabbits proved to be successful. Since that time, vaccines have been developed for many diseases, including diphtheria, polio, pertussis (whooping cough), measles, mumps, rubella (German measles), hepatitis, and influenza. Vaccines are made from either weakened live or killed microorganisms. When introduced into the body, they stimulate the production of antibodies, providing active immunity against bacterial and viral diseases.

Monoclonal antibodies

Monoclonal ( mono means "one") antibodies are identical antibodies produced by clones (exact copies) of a single cell. The cell from which the clones are made is created by combining a B cell containing a specific antibody with a myeloma (a form of cancer) cell. The resulting hybrid produces the specific antibody of the parent B cell and divides indefinitely like the parent cancer cell. Clones of the hybrid cell produce virtually unlimited amounts of one type, or monoclonal, antibodies. Monoclonal antibodies are used in many medical diagnostic tests, such as pregnancy tests, and in the treatment of cancer and other diseases.

[ See also Allergy ; Blood ; Immune system ; Rh factor ; Transplant, surgical ; Vaccine ]