An antigen is a tiny piece of a substance — often a protein on a virus, a bacterium, or another cell — that the body reads as not me. When the immune system notices it, it can make antibodies, which are tools shaped to fit that antigen. So the antigen is the trigger; the antibody is the reply. You meet antigens all the time: on germs, on pollen, on some foods. Blood types are antigen differences on red blood cells (A and B are names for antigens). A vaccine puts in antigens, or a safe copy of them, so your body can practise making antibodies without you having the full illness. Though actually not every antigen is an enemy. Your own cells have antigens too; the body is supposed to leave those alone, and trouble starts when it does not, as in some autoimmune illness. The word looks as if it contained gene, as in DNA; say it AN-ti-jen. The -gen piece is an older scientific bit meaning that which produces, taken in from French.