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colloid substances of high molecular weight which, when introduced into the bodies of animals and man, stimulate the formation of specific antibodies that react with them. An indispensable condition for antigenicity is the difference of the antigen from the substances normally present in the recipient. Antigens consist principally of foreign proteins, some polysaccharides (mostly of bacterial origin), and complexes of proteins and various chemical compounds. There are corpuscular antigens—for example, suspensions of bacteria—which produce agglutination (clumping) reactions with antibodies and soluble antigens—for instance, toxins—which produce precipitation (sedimentation) reactions. Simple proteins of low molecular weight, such as gelatin proteins, are not antigenic. Egg and blood serum albumens (molecular mass 40,000–70,000) have less antigenicity than gamma globulins and other proteins with higher molecular mass (> 160,000). Lipides and carbohydrates that have no antigenic properties and that acquire them in combination with proteins are called haptens. Antigens may be synthesized artificially by combining any of a number of chemical substances with protein. Antigens contain two components, a substance of high molecular weight which is the vehicle for antigenic stimulation (usually a protein) and a nonprotein group whose structure determines the specificity of the given antigen. These groups are called determinants (or specificity factors); they are located on the surface of the antigen and may be separated from the colloid carrier. In certain pathological states—for instance, hemolytic anemia—the body’s own proteins acquire antigenic properties, that is, they become autoantigens. Since proteins have individual specificity, proteins of an animal are antigens for other animals of the same species (isoantigens). It is precisely for this reason that reactions resulting from tissue incompatibility occur in transplants (except in cases of transplant from one monovular twin to another).