Answer: Horseshoe Crabs

Horseshoe crabs have blood with a unique property found in their genus and select other marine invertebrates: their blood contains a cell known as an amebocyte that serves as a defensive measure much like white blood cells in mammals.

The amebocytes found in horseshoe crab blood have proved invaluable in modern medical diagnostic work because of their simple and reliable action. An aqueous extract of the amebocytes known as Limulus amebocyte lysate (LAL) reacts strongly with bacterial endotoxins (even if the host bacteria is dead). As such, a blood sample of the patient can be mixed with the LAL and if it congeals (the reaction of the amebocyte to the endotoxins), it’s indicative of gram negative bacterial infection in the patient.

The process was first discovered by Fred Bang back in 1956 but was not approved by the FDA until 1970; since then the diagnostic test has been widely used and has completely replaced earlier methods (which relied on exposing a lab animal to the patient’s blood).