The blue blood that guards every hospital on Earth
Horseshoe crab blood is blue. Not metaphorically, literally blue, because it uses copper-based hemocyanin to carry oxygen instead of the iron-based hemoglobin that makes human blood red. But the colour is not why the biomedical industry harvests roughly 600,000 horseshoe crabs every year along the Atlantic coast of the United States. The reason is a specialised immune cell called the amebocyte, which the crab's blood is packed with.
