Allergic reactions to bee stings can be damaging or even deadly, but new evidence from two independent studies of mice reported in the Cell Press journal Immunity on October 24th suggest that the immune response to bee venom and other allergens actually evolved and may continue to serve as a protective defense mechanism. Perhaps they aren't just misdirected immune responses after all.
"Our study adds to the argument that allergy evolved to protect us from noxious factors in the environment - it protects us by making us sneeze, cough, vomit, and itch, by inducing a runny nose and tears," said Ruslan Medzhitov of Yale University School of Medicine. "All of these reactions are designed to expel something harmful from the body. They are unpleasant, but they protect by being unpleasant."
"Everyone who ever witnessed or even experienced an anaphylactic reaction to a bee or a wasp sting will wonder why evolution did not get rid of such a potentially deadly immune reaction," added Martin Metz of Charité-Universitätsmedizin Berlin. "We have now shown in mice that the development of IgE antibodies to honeybee venom and also to the venom from a poisonous snake can protect mice to some degree from the toxic effects of the venoms."
It is apparently only when allergic reactions run amok that they cause serious problems.
Metz and his colleague Stephen Galli of Stanford University School of Medicine found that mice injected with amounts of honeybee venom similar to that which could be delivered in one or two stings developed a specific immune response, which subsequently increased their resistance to potentially lethal amounts of venom. The researchers observed a similar protective immune response in the mice following exposure to poisonous snake venom. In both cases, that protective effect was attributed to IgE antibodies, which are produced in response to a broad range of environmental antigens, many of them seemingly harmless.
The common venom ingredient and major allergen in bee venom, PLA2 (phospholipase A2), is an enzyme that wreaks havoc by destroying cellular membranes. In the second study, Medzhitov and his colleagues showed how PLA2 induces the type 2 immune response in exposed mice, to afford the animals later protection against near-lethal doses of damaging enzyme.
It seems as though our bodies might know what they are doing after all. But, if immune reactions to bee stings are advantageous, why then do some people develop anaphylaxis?
"We don't know," Galli said, "but perhaps only certain people, who for genetic or other reasons exhibit especially severe IgE-dependent reactions, are at risk for developing anaphylaxis when stung by bees. This notion is supported by clinical observations showing that only a small fraction of people who have IgE antibodies against honeybee venom develop anaphylaxis upon being stung by a bee."
Immunity, Marichal et al.: "A Beneficial Role for Immunoglobulin E in Host Defense against Honeybee Venom."
Immunity, Palm et al.: "Bee venom phospholipase A2 induces an ST2-dependent type 2 response and Immunoglobulin E-mediated protective immunity."