A toxoid is a bacterial toxin (usually an exotoxin) whose toxicity has been inactivated or suppressed either by chemical (formalin) or heat treatment, while other properties, typically immunogenicity, are maintained. Thus, when used during vaccination, an immune response is mounted and immunological memory is formed against the molecular markers of the toxoid without resulting in toxin-induced illness. In international medical literature the preparation also is known as Anatoxin or Anatoxine. There are toxoids for prevention of diphtheria, tetanus and botulism.
Toxoids are used as vaccines because they induce an immune response to the original toxin or increase the response to another antigen since the toxoid markers and toxin markers are preserved. For example, the tetanus toxoid is derived from the tetanospasmin produced by Clostridium tetani. The latter causes tetanus and is vaccinated against by the DTaP vaccine. Botulin is produced by Clostridium botulinum and it causes the deadly disease botulism. While patients may sometimes complain of side effects after a vaccine, these are associated with the process of mounting an immune response and clearing the toxoid, not the direct effects of the toxoid. The toxoid does not have virulence, as did the toxin before inactivation.
Multiple doses of tetanus toxoid are used by many plasma centers in the United States for the development of highly immune persons for the production of human anti-tetanus immune globulin (Tetanus Immune Globulin (TIG), HyperTet (c)), which has replaced horse serum type tetanus antitoxin in most of the developed world.