Antibody
Antibodies are protein molecules that function in the body's immune response. They are present throughout the circulatory and lymph systems, and are therefore exposed to all tissues in the body. An antibody is able to recognize and bind to a particular offending antigen. Antigens stimulate immune responses because they are recognized to be foreign, or "non-self." Invaders such as bacteria, viruses, fungi, toxins, and other foreign substances generally carry a variety of antigens on their surfaces.
The antibody for a particular antigen functions by binding to that antigen. This results in one of two possibilities. The antibody may deactivate the antigen by either blocking its active site or otherwise changing it so that it can no longer harm host cells. Alternatively, an antibody may label the antigen-carrying object for destruction. In this case, one part of the antibody binds to the antigen while another part binds to immune system cells that are specialized to destroy antigens, cells such as macrophages or neutrophils.
Foreign organisms such as bacteria or viruses typically possess numerous antigens on their surfaces. In addition, any particular antigen can usually be recognized by numerous antibodies, each of which binds to a slightly different site on the antigen. Each part of an antigen that can be bound by an antibody is called an epitope. With multiple epitopes on each antigen, and multiple antigens for any foreign invader, numerous antibodies can potentially be involved in an immune response.
Antibodies are made by immune system cells known as B-lymphocytes. B-lymphocytes are produced in the red marrow of bones. After they mature, the cells move to lymph nodes and begin to secrete antibodies into the lymph and blood. Each B-lymphocyte cell produces a unique antibody that targets a specific antigen.
Antibodies are Y-shaped proteins with binding sites at the tips of the branches of the Y. The antibody binds to an antigen in a way similar to how a key fits into a lock. The site on the antibody that binds to the antigen is known as the Fab region.
The antibody protein bundle contains two pairs of chains of proteins held together by disulfide bonds. The two identical longer chains, called heavy chains, form the base of the Y and one-half of each branch of the Y. The two identical shorter chains, called light chains, form the other halves of the branches of the Y. The ends of the branches of the Y contain a variable region on both the heavy and light chains. These are the Fab regions.
There is great diversity in Fab regions, which is essential to the body's ability to respond to a wide range of antigens. High diversity is possible because each heavy and light chain consists initially of numerous different segments, which can be spliced and combined in a variety of different ways. Consequently, there are thousands of possible heavy chains and light chains, with each giving rise to a slightly different binding site.
Antibodies are divided into five different classes. IgA antibodies function at mucous-producing surfaces such as the bronchioles, nasal passages, vagina, and intestine. They are also present in saliva, tears, and breast milk.
Antibodies are made by immune system cells known as B-lymphocytes. Redrawn from Hans and Cassady.The function of antibodies of the IgD group is unclear. Most of these antibodies are not secreted into the bloodstream but, rather, are associated with B-lymphocytes.
IgE antibodies are found at mucous-producing surfaces, as well as in blood and tissues. They are responsible for many hypersensitive, or allergic, responses, in which the immune reaction to a relatively unharmful antigen is disproportionately intense. IgG antibodies are abundant in the bloodstream. They are able to cross the placenta and therefore provide the only protection for babies until their own immune systems mature. IgG antibodies are a very active antibody group that also plays a role in neutralizing toxins. IgM antibodies are largely found on B-lymphocytes.
Medical Uses of Antibodies
Vaccinations against various diseases are often made using antigens isolated from bacteria or viruses. Removed from their carriers, these antigens are in and of themselves harmless. However, they still trigger an immune response, after which antibodies specific for those antigens continue to circulate in the bloodstream. This allows those antibodies to be produced quickly and in great quantity in case of a future invasion by the entire pathogen.
Antibody-binding activity can also be used to diagnose disease. That is how HIV infections are identified.
In addition, attempts have also been made to produce antibody-related therapies for cancer. These aim to take advantage of the great specificity of antibodies to fight tumors. Some scientists are optimistic about the use of monoclonal antibodies in cancer therapy. These are antibodies that are specifically designed to recognize molecules present in tumor cells but not in healthy cells.
These antibodies can then be used to target antigens that are present only in small quantities, as is the case with many cancer cells. Monoclonal antibodies can function on their own by tagging cancerous cells for destruction, or can be attached to toxins or radioisotopes that help to destroy cancer cells.
Cancer cells do not typically induce an immune response in the host because they are not foreign. However, they can be transplanted to another organism, such as a mouse, where an immune response can be induced. After antibodies are harvested from the reaction, monoclonal antibodies can be isolated and cloned.
Bibliography
Curtis, Helena. Biology. New York: Worth Publishers, 1989.
Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W. W. Norton, 1996.
Kuby, Janis. Immunology, 3rd ed. New York: W. H. Freeman, 1997.
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