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.