The giant keyhole limpet's hemolymph carries a protein that is the essential component of a new cancer vaccine. Keyhole limpet hemocyanin (KLH) carries oxygen in limpet blood. It is an unusually large protein—near virus size—and contains many epitopes, which trigger our body to produce antibodies. When doctors inject KLH into the human bloodstream, it provokes a powerful immune response. If markers for a certain cancer are attached to KLH, the immune system can be stimulated to attack them. Unlike some synthetic alternatives, KLH is nontoxic. Researchers use the protein in cancer vaccines to "break tolerance," says Frank Oakes, the CEO of Stellar Biotechnologies, which grows limpets in a business park for aquaculture next to the Pacific Ocean in Port Hueneme, California. "Your body tolerates the cancer cell because the body believes it is a part of you," he says.