Every Vaccine in America Is Tested With Horseshoe Crab Blood

In the early 1950s, Frederik Bang was studying horseshoe crab circulation at the Marine Biological Laboratory in Woods Hole when one of his crabs died of a bacterial infection. Bang opened it up and found something strange: nearly all the blood inside the crab had clotted into a single semi-solid mass. Healthy horseshoe crab blood, drawn into a tube, stays liquid. This one had locked up.

The clot was a defense system. Limulus polyphemus, a 450-million-year-old animal with no antibodies and barely any immune system to speak of, deals with bacteria by sealing them inside a coagulated cage. The trigger is endotoxin — a molecule on the outer wall of gram-negative bacteria. Bang and the hematologist Jack Levin spent the 1960s isolating the agent, a single cell type called the amebocyte, and working out how to use its lysed contents as a detector.

The result, called LAL (Limulus amebocyte lysate), was approved by the FDA in 1977 for testing every injectable drug, vaccine, and implanted device in the United States. If endotoxin is present at parts per trillion, LAL clots. The slower rabbit pyrogen test it replaced took days and a lot of rabbits.

The blood is blue, by the way, because horseshoe crabs run on copper-based hemocyanin instead of iron-based hemoglobin. Oxygenated copper is blue. A synthetic recombinant Factor C — the active enzyme cloned out of the cascade — was finally given equal pharmacopeial standing in May 2025, after twenty-two years on the market and roughly half a million crabs bled per year in the meantime.