ALLERGY, a hypersensitivity which causes various diseases such as serum sickness, asthma, hay fever and hives. When a foreign protein, say horse serum, is injected into a laboratory animal of another species, it is harmless. But it renders the animal hypersensitive to further injections of the same serum after a period of incubation. When the second injection is given the animal will have a severe reaction which may result in its death. This sensitivity is remarkably specific, and the animal will react only to the type of foreign protein originally given. One hypothesis is that the tissues form antibodies to counteract the original foreign protein and when the second injection is made the antibodies and foreign protein react so violently that de struction of tissue takes place. This has never been experimentally proved.