An antigen is the specific part of a pathogen (or potential or suspected pathogen, in the case of autoimmune disease) that the immune system recognizes as a threat and produces antibodies to bind to. Any number of molecules can be an antigen, including proteins, peptides, polysaccharides, lipids, or nucleic acids. In vaccine development, choosing the appropriate antigen is crucial to invoke an adequate, lasting immune response against the pathogen. In COVID-19 vaccines, the antigen is the spike protein. A similar term, epitope, refers to the specific part of an antigen molecule that the antibody attaches itself to.
