An antigen is any substance or organism that is unrecognized by our immune system. Any foreign invaders, such as pathogens (bacteria and viruses), chemicals, toxins, and pollens, can be antigens. Under pathological conditions, normal cellular proteins can become self-antigens. An antigen contains distinct sites on its surface, which is called an epitope or antigenic determinant. Antibodies generated against an antigen recognize and interact (in fact, the name is derived from “antibody generators) with specific epitopes via antigen-binding sites

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