Antibody proteins are also known as immunoglobulins. They are found in the gamma globulin class of plasma proteins, as identified by a technique called electrophoresis in which different types of plasma proteins are separated by their movement in an electric field (fig. 15.9). The five distinct bands of proteins that appear are albumin, alpha-1 globulin, alpha-2 globulin, beta globulin, and gamma globulin.
The gamma globulin band is wide and diffuse because it represents a heterogeneous class of molecules. Since antibodies are specific in their actions, it follows that different types of antibodies should have different structures. An antibody against smallpox, for example, does not confer immunity to poliomyelitis and, therefore, must have a slightly different structure than an antibody against polio. Despite these differences, antibodies are structurally related and form only a few classes.
There are five immunoglobulin (abbreviated Ig) subclasses: IgG, IgA, IgM, IgD, and IgE. Most of the antibodies in serum are in the IgG subclass, whereas most of the antibodies in external secretions (saliva and milk) are IgA (table 15.6). Antibodies in the IgE subclass are involved in certain allergic reactions.
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