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Any foreign material—usually of a complex nature and often a protein—that binds specifically to a receptor molecule made by lymphocytes is called an antigen. Antigens include molecules found on invading microorganisms, such as viruses, bacteria, protozoans, and fungi, as well as molecules located on the surface of foreign substances, such as pollen, dust, or transplanted tissue. When an antigen binds to a receptor molecule, it may or may not evoke an immune response. Antigens that induce such a response are called immunogens. Thus, it can be said that all immunogens are antigens, but not all antigens are immunogens. For example, ...(100 of 13540 words)