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Antibodies (Immunoglobulins)
Antibodies are highly specific proteins that are created by specialized white blood cells called plasma cells as a result of B-cell interaction with pathogenic antigens. These antibodies can either be embedded in the membrane of white blood cells or they can be found floating around in the blood plasma or lymph system. Either way, once these antibodies bind to their specific antigen, they label the antigen for destruction by our immune system. Every antibody consists of four polypeptide subunits (two heavy chains and two light chains) that are connected via disulfide bridges to form a Y-shaped structure. The variable section of the antibody contains the proper sequence of amino acids that can bind to the antigen and this is called the antigen-binding site. The site on the antigen itself that binds to the antibody is called the antigenic-determinant or epitope. The variable section is so called because it varies from one antibody to another, which makes sense because different antibodies must be able bind to different antigens. The other portion of the antibody is called the constant portion. This segment remains the same within the same class of antibodies (there are five different classes) and it can be used to bind onto the membrane of immune cells. Once the antibody binds onto the antigen, it can (1) cause the process of agglutination (2) directly inactivate the antigen (3) call upon white blood cells such as macrophages to destroy the pathogen agent. Antibodies, also called immunoglobulins (Ig) come in five types of classes (IgG, IgE, IgM, IgD and IgA). These classes differ from one another not only in their constant region but also in their mechanism by which they defend our body.
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