Horseshoe crab blood contains a key protein that defends against impurities, which is used in medical research when testing medical instruments, implants and pharmaceuticals. Their ability to protect themselves against impurities while not having an immune system allows horseshoe crabs to live in bacteria-saturated environments. Anything that touches blood is tested using aqueous extract of blood cells from the horseshoe crab, which is used for the detection and measurement of bacterial endotoxins.