A foreign substance or infection, which stimulates an immune antibody response. Antigens have the potential to cause disease, unless they are inactivated antigens as used in vaccines to elicit an immune protective response. Each antigen is characterised by its specific individual structure that identifies it as being foreign to the body. Antigens are present on infectious viruses, bacteria, fungi, protozoa, and toxins. They are found either circulating freely in the blood or are displayed by virus-infected cells.