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The mollusk Megathura crenulata produces a protein that scientists use as part of vaccines and other medical applications. This protein is called keyhole limpet hemocyanin because the mollusk's common name is the giant keyhole limpet. The important feature of keyhole limpet hemocyanin is that it interacts well with human immune systems, which helps spark immune responses to treatments such as vaccines.
Hemocyanin comes from the Greek words haima, meaning blood, and kyanos, meaning blue pigment. The keyhole limpet is named for its appearance, as the mollusk is large, round, and flat with a central dark "keyhole." The compound hemocyanin is not found in many organisms, only those that are arthropods or mollusks.
In the limpet, hemocyanin is an oxygen-carrier protein. The protein contains two copper atoms at each site that help bind an oxygen molecule. The structure of the protein is split into subunits, and it can break down into individual subunits under certain conditions. Normally, the protein moves around the blood-lymph system of the limpet to collect and transport oxygen.
Keyhole limpet hemocyanin has a special ability to interact with the vertebrate immune system. For this reason, scientists have adopted it as a carrier protein for other substances that they want to make the human body recognize. T-cells, macrophages, polymorphonuclear lymphocytes, and monocytes all react to the protein. This recognition of the foreignness of the protein sets a cascade of reactions in motion.
Vaccines are an example of the use to which keyhole limpet hemocyanin may be put. The basis of vaccination is that someone who hasn't yet come into contact with a disease is protected from its worst effects if his or her immune system is set up to recognize it and has the appropriate immune responses already in place to deal with the infection. Therefore, scientists inject part or all of an infectious organism into the body so it can practice dealing with the infection under safe conditions.
This form of immunization may elicit a strong or a weak response, depending on the antigenicity of the chosen vaccine molecules. For vaccines that do not elicit a strong enough response from the immune system, scientists can attach the keyhole limpet hemocyanin. In this way, the body recognizes the hemocyanin strongly and the attached vaccine molecule as well. If the vaccinated person then comes into contact with the infectious organism, his or her immune system will react more effectively to the threat. Another application of the protein is as an adjunct to antibiotic treatment using the same principles of immune responses.