What Is an Antibody ? Types of Antibodies Features Antibodies are composed of four polypeptide chains: two light chains and two heavy chains. These form a molecule called an antibody monomer. One light chain and one heavy chain line up with each other and form a mirror image with the other two chains, creating the shape of a Y or T. The heavy chain is approximately twice as long as the light chain. Both chains have a constant portion and a variable portion. The variable portions of the light and heavy chains match up at the top of the Y or T and form the antigen binding site. Each antigen binding site is antigen- specific. The constant regions are relatively the same across immunoglobulin classes. IgM IgM has a monomer form and pentamer form, which is five linked monomers. The IgM monomer usually is bound at the surface of a B cell and acts as the cells antigen binding site. The pentamer circulates freely in the bloodstream upon release from a plasma cell. The IgM pentamer is the first antibody released by the body during the primary immune response. This makes IgM diagnostically useful in studying the stage of infection. IgA IgA is found in monomer form and dimer form, or two linked monomers. It is found in normal body fluids and excretions like sweat, saliva, milk and intestinal juice. These antibody-laden excretions protect the epithelial cells of the skin on which they are produced from bacterial infection. IgD IgD is a monomer. IgD is almost universally found attached to a B cell and acts as the antigen binding site. The IgD class of antibodies are important in B cell activation. IgG The IgG class of antibodies are monomers that comprise the majority of antibodies in circulation. IgG antibodies can be found in blood and lymph fluid and is able to cross the placenta to impart passive immunity from mother to fetus. The IgG antibodies can protect the body against viruses, bacteria and other toxins. They also are strong complement fixers. Complement fixation is a method of inducing lysis, or break-up, of a foreign cell. IgE IgE antibodies are monomers found in high levels during an allergic reaction. IgE signals the release of histamine and other inflammatory chemicals upon antigen binding. IgE is released by skin cells, tonsils and cells that line the respiratory and digestive tracts.
Posted on: Sat, 28 Jun 2014 14:30:57 +0000
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