Horseshoe crab research. Researcher using a syringe to take blood from a horseshoe crab (Limulus polyphemus). The horseshoe crab, contrary to its name, is a distant relative of the spider. It has existed unchanged for over 500 million years. Its immune system is primitive but powerful. Two proteins (alpha2-macroglobulin and limulin) in its blood are thought to be respons- ible for its entire immune response to invading micro-organisms. Similar proteins are found in human blood, and the research may provide an insight into the human immune system. Photographed at the Woods Hole Marine Biology Laboratory, Massachusetts, USA, in 2002.