Any of various foreign substances — such as bacteria, viruses, endotoxins, exotoxins, foreign proteins, pollen and vaccines — whose entry into an organism induces an immune response — antibody production, lymphokine production, or both — directed specifically against that molecule. Response may be demonstrated as an increased reaction, such as hypersensitivity (usually protein or a complex of protein and polysaccharide, or occasionally a polysaccharide of high molecular weight), a circulating antibody that reacts with the antigen, or some degree of immunity to infectious disease if the antigen was a microorganism or its products.