toxoid, protein toxin treated by heat or chemicals so that its poisonous property is destroyed but its capacity to stimulate the formation of toxin antibodies, or antitoxins, remains. Because toxoids can be given in large quantities with no risk of tissue damage, they have superseded the highly poisonous toxins as immunizing agents against such diseases as diphtheria and tetanus.

Bacterial toxin that has been made inactive but can still combine with or stimulate formation of antibodies. In many bacterial diseases, the bacteria produce a toxin that causes the disease manifestations. Heating the toxin or treating it chemically converts it into a harmless toxoid that can be injected into a human or a nonhuman animal to confer immunity from subsequent infection. The vaccines for tetanus and diphtheria are toxoids.

Learn more about toxoid with a free trial on Britannica.com.