Pharmacology · Immunoglobulins and antitoxins
Diphtheria antitoxin (equine) !
Horse antibodies that neutralise circulating diphtheria toxin. Given with antibiotics on clinical suspicion, without waiting for the laboratory.
Updated 2026-11-01
Also called: Diphtheria antiserum, DAT
The figure shows monomeric diphtheria toxin from Corynebacterium diphtheriae, which the antitoxin neutralises.
Mechanism of action
Diphtheria toxin enters cells and inhibits protein synthesis; it damages local tissue (pseudomembrane), the heart and the nerves. The antitoxin binds circulating toxin and neutralises it, halting progression. It does not act on toxin already fixed to tissues, so timing is decisive. Antibiotics stop further toxin production.
- Toxigenic bacteria release toxin in the pharynx
- Equine antitoxin binds the circulating toxin
- Neutralised toxin cannot enter heart or nerve cells
- Antibiotics stop the bacteria and toxin production
Pharmacokinetics
- Distribution
- Equine protein in plasma
- Half-life
- Animal-derived antibodies are cleared faster than human IgG
Indications
- Suspected or confirmed respiratory diphtheria, together with antibiotics and as early as possible
Adverse effects
Common
- Fever and reaction at the administration site
Contraindications
- No absolute contraindication in severe diphtheria; allergy to equine proteins requires readiness to treat anaphylaxis
Key interactions
- Antibiotics
- Complementary: the antitoxin neutralises toxin and the antibiotic eliminates the bacterium
What is monitored
- Airway and extent of the pseudomembrane
- ECG and signs of myocarditis
- Signs of anaphylaxis during administration and of serum sickness afterwards
Administration and safety
- Given in hospital with everything needed to treat anaphylaxis
- Equine product: ask about previous reactions to horse sera
- In some countries it is only obtained through public health programmes
Nursing care
- Apply the isolation precautions of the local protocol
- Monitor the airway and heart rhythm
- Watch for urticaria, bronchospasm or hypotension during administration
- Explain that fever, hives or joint pain may appear days later
Pregnancy and breastfeeding
In pregnancy it is considered as in other patients, because severe diphtheria endangers mother and fetus.
Test yourself
Think of the answer, then tap the question to see it.
Why is diphtheria antitoxin given without waiting for laboratory confirmation?
Because it only neutralises circulating toxin; toxin fixed to tissue cannot be reversed, and delay allows more damage.
Which animal is diphtheria antitoxin made in, and what reactions does it cause?
The horse; it can cause anaphylaxis and, days later, serum sickness.
Which treatment always accompanies the antitoxin?
Antibiotics, which eliminate the bacterium and stop toxin production, followed by vaccination on recovery.
Related drugs
← Back to the class: Immunoglobulins and antitoxins
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Chapter 7: Diphtheria | Pink Book | CDC (Centers for Disease Control and Prevention, 2021); Diphtheria (World Health Organization, 2026); Farmacología básica y clínica (Katzung), appendix (vaccines and immune globulins) (McGraw Hill, 2025); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).
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Representative model, for educational purposes only.