Toxoid
Summary
A toxoid is a bacterial exotoxin that has been chemically inactivated, usually with formaldehyde, so it retains antigenicity but not toxicity. Toxoid vaccines generate neutralizing antibody against the toxin rather than against the organism.
Detail
The tetanus and diphtheria vaccines are the classic toxoids, and they exemplify an important principle: in these diseases the toxin, not bacterial invasion, causes the illness, so antitoxin antibody prevents disease without preventing colonization. Because immunity wanes, a booster is needed every ten years, or every five years for a tetanus-prone wound; wound management combines the toxoid with tetanus immune globulin for a contaminated wound in an incompletely immunized patient, which is the standard example of passive plus active immunization given together. Acellular pertussis vaccine also contains inactivated pertussis toxin alongside filamentous haemagglutinin and pertactin. Toxoids are inactivated and therefore safe in pregnancy and immunodeficiency, and Tdap is recommended in every pregnancy between twenty-seven and thirty-six weeks so that transplacental antibody protects the newborn. Other vaccine categories worth contrasting are live attenuated (MMR, varicella, rotavirus, yellow fever, intranasal influenza, BCG), inactivated whole organism, subunit, and conjugate vaccines, the last linking polysaccharide to protein to elicit T-cell-dependent memory in infants.
Sources
- Janeway's Immunobiology
- First Aid for the USMLE Step 1
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