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← The ApothecaryLimulidaeblood (amebocytes, source of Limulus amebocyte lysate)Fact-checked 23 October 2026

Atlantic horseshoe crab

Limulus polyphemus

Also calledHorseshoe crab · Limulus

An ancient marine arthropod whose blue blood clots on contact with bacterial endotoxin. Limulus amebocyte lysate (LAL), made from that blood, has been the standard test since the 1970s for making sure vaccines, injectable drugs and implants are free of fever-causing endotoxins. It is not a treatment, and its harvest kills a share of the crabs bled; a synthetic alternative now exists.

Full natural-history sheet →

What it is used for

  • Fever
    Strong

    LAL detects bacterial endotoxins, which cause fever and shock if injected, so every batch of injectable drugs, vaccines and implanted devices is tested with it or an approved alternative; it replaced the slower rabbit pyrogen test after FDA approval in 1977. This protects patients indirectly; no one takes horseshoe crab blood as medicine.1,2

Safety

Toxicity

Not taken as a medicine. Horseshoe crabs are not dangerous to handle; the tail is used for righting, not stinging.

In the healing traditions

  • Modern pharmacology

    Limulus amebocyte lysate (LAL); recombinant Factor C (rFC)

    Endotoxin testing of pharmaceuticals and devices. rFC, made in insect cells without crabs, has been available since 2003, entered the European Pharmacopoeia in 2016 and gained its own USP chapter (<86>) in 2025.

Active compounds

Factor C
serine protease that triggers the clotting cascade on binding endotoxin; recombinant version replaces LAL
coagulogen
clotting protein that forms the gel
haemocyanin
copper-based oxygen carrier that makes the blood blue

History

In 1956 Frederik Bang at Woods Hole found that bacteria made horseshoe crab blood clot; with Jack Levin he traced the reaction to endotoxin and the amebocyte cells. LAL was approved by the FDA in 1977; the crabs are caught, bled at licensed facilities and released.

Sources

  1. 1.Wikipedia: Limulus amebocyte lysate (discovery, approval, rFC, mortality, USP chapters)
  2. 2.Iwanaga S. Biochemical principle of Limulus test for detecting bacterial endotoxins. Proc Jpn Acad Ser B 2007
  3. 3.Maloney T et al. Saving the horseshoe crab: a synthetic alternative to horseshoe crab blood for endotoxin detection. PLoS Biol 2018
  4. 4.Leschen AS, Correia SJ. Mortality in female horseshoe crabs (Limulus polyphemus) from biomedical bleeding and handling. Mar Freshw Behav Physiol 2010
  5. 5.Smith DR et al. Limulus polyphemus. IUCN Red List of Threatened Species 2016
  6. 6.Johns Hopkins Bloomberg School of Public Health: Frederik Bang (Heroes of Public Health)

Reference, not prescription. Nothing here is medical advice. Herbs can interact with medicines and be dangerous in pregnancy; tell your doctor or pharmacist about anything you take.