Choose the answer that best completes the blanks of this sentence in the correct order. are substances that provoke a specific immune response which is so discriminating that only a single molecular fragment, called an actually interacts with the lymphocyte's receptor

The correct answer is antigens and epitope.
An antigen is a substance which triggers an immune response in an organism. This can be either a foreign substance, such as a pathogen, or a specific part of the host organism (in this case this is an autoimmune response). The presence of an antigen activates a specific part of the immune system, called antibody. Each antigen has a specific antibody, which is tailored by the immune system accordingly. More specifically, the antibody has a specific structure, called paratope, which is complementary and binds like a key to a specific structure of the antigen, called epitope. 

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