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# 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

- Class: [Immunoglobulins and antitoxins](https://durgical.com/en/pharmacology/classes/immunoglobulins-and-antitoxins/)
- Target: Circulating diphtheria toxin
- ATC code: J06AA01
- WHO Model List of Essential Medicines

Also called: Diphtheria antiserum, DAT

◆**Circulating diphtheria toxin**Diphtheria antitoxin (equine)

ⓘ The figure shows monomeric diphtheria toxin from Corynebacterium diphtheriae, which the antitoxin neutralises.

**Target in 3D:** Circulating diphtheria toxin. PDB structure 1MDT (RCSB PDB, public domain). [View the structure at RCSB PDB](https://www.rcsb.org/structure/1MDT) · Drag to rotate · pinch to zoom

## 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

### Serious (rare, important)

- Anaphylaxis (horse protein)
- Serum sickness days later

## 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.

## Memory aid

Diphtheria antitoxin = "before the lab": toxin that becomes fixed can no longer be neutralised.

## 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.

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## Related drugs

- [Diphtheria vaccine (diphtheria toxoid)](https://durgical.com/en/pharmacology/diphtheria-vaccine/)

[← Back to the class: Immunoglobulins and antitoxins](https://durgical.com/en/pharmacology/classes/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](https://www.cdc.gov/pinkbook/hcp/table-of-contents/chapter-7-diphtheria.html) (Centers for Disease Control and Prevention, 2021); [Diphtheria](https://www.who.int/news-room/fact-sheets/detail/diphtheria) (World Health Organization, 2026); Farmacología básica y clínica (Katzung), appendix (vaccines and immune globulins) (McGraw Hill, 2025); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=J06AA01) (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/diphtheria-antitoxin/