Nearly every vaccine on Earth is tested using a crab's baby-blue blood
Horseshoe crab blood runs blue because it carries copper-based hemocyanin instead of iron-based hemoglobin, and it clots almost instantly on contact with bacterial toxins. That reaction became the basis of the Limulus Amebocyte Lysate (LAL) test, the standard worldwide method for confirming that injectable drugs and vaccines are free of bacterial contamination.