An antigen is an immunogenic substance produced in the body. Antigens are substances that stimulate the immune system in vivo or in vitro to generate an immune response, produce antibodies and lymphocytes, and bind specifically to the corresponding antibodies and lymphocytes. It leads to the proliferation and differentiation of immune cells and the production of immune response products, including antibodies and sensitized lymphocytes. These substances produce specific immune effects and remove antigens that invade the body. Depending on their chemical nature, antigens can be categorized as polysaccharides, proteins, nucleic acids, peptide oligosaccharides, small molecule chemicals, lipids, etc., with most antigens being proteins. Meanwhile, the specificity of an antigen is determined by the antigenic determinant clusters on its surface, and the nature and location of its epitope structure may affect the specificity of the antigen. The presence of antigens protects the body’s immune system and contributes to human immunity.