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 Because toxoids can be given in large quantities with no risk of tissue damage, they have superseded the highly poisonous toxins as immunizing agents against such diseases as diphtheria and tetanus.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.

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A toxoid is a bacterial toxin (usually an exotoxin)whose toxicity has been weakened or suppressed either by chemical (formalin) or heat treatment, while other properties, typically immunogenicity, are maintained. Toxoids are used in vaccines as they induce an immune response to the original toxin or increase the response to another antigen. For example, the tetanus toxoid is derived from the tetanospasmin produced by Clostridium tetani and causing tetanus. The tetanus toxoid is used by many plasma centers in the United States for the development of plasma rich vaccines.

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