Horseshoe crab blood is primarily used in medicine to detect bacterial endotoxins in vaccines, drugs, and medical devices to ensure their safety for human use.

the blood contains cells called amebocytes, which react and clot in the presence of endotoxinsbacterial substances that can cause severe illness or death if introduced into the human bloodstream. this clotting property led to the development of Limulus Amebocyte Lysate (LAL), a test derived from horseshoe crab blood, which is the standard assay worldwide for detecting endotoxins in medical products.

Horseshoe crabs are collected from the wild, bled by extracting a portion of their blood (up to 30% or possibly more according to some accounts), and then typically returned to their habitat. this blood harvest has significant biomedical value, with the blood sometimes described as extremely expensive