n. A substance which neutralizes the action of a toxin or poisonous ptomaine; specifically, a substance developed in the body of an animal by inoculation with the germs of diphtheria, which enables it to tolerate this poison.n. A substance which neutralizes the action of a poison.n. Specifically: The antibody to a toxin of bacterial or related origin which is produced as the result of immunization with the corresponding toxin. Examples are the diphtheria antitoxin and the tetanus antitoxin.n. In botany, a substance secreted by a plant which protects it against destructive microbes.