Why on Earth would human evolution hang onto allergic reactions to bee stings – especially ones that could kill you? Perhaps because the same things that lead to anaphylaxis might also help you build up defenses against toxins.

Most allergic reactions are kickstarted by an antibody called Immunoglobulin E and involve immune cells called mast cells. Sometimes IgE can send the immune system into overdrive, causing a severe allergic reaction that results in everything from inflammation to hives to even cardiac arrest. Until now, scientists haven’t found a good explanation for why we hang onto this antibody that seems determined to end us. Now, two new studies in mice, published Thursday in the journal Immunity, have found that IgE could be an essential component of our body’s defenses against venomous nasties.

"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," Ruslan Medzhitov of Yale University School of Medicine said in a statement. "All of these reactions are designed to expel something harmful from the body. They are unpleasant, but they protect by being unpleasant."

Scientists found that when mice are injected with the amount of honeybee venom that would be delivered by one or two stings, they subsequently develop higher resistance levels to it. The mice that got the initial sting were more protected against a second dose of venom – equivalent to about five bee stings -- that could be potentially fatal. Compared to control mice that received a dummy shot of salt solution for the first part of the experiment, the venom-dosed mice didn’t get as sick and were three times likelier to survive.

The researchers confirmed that IgE is key to the process by running their experiments in mice bred to lack functional IgE or mast cells. All of the mutants did not get any benefit from the initial venom dose. IgE and mast cells hold the key to the protective effect. Severe allergic reactions are just an extreme example of what happens when this immune process goes wrong, the scientists say.

The IgE-driven immune response probably “developed, at least in evolutionary terms, to protect the host against potentially toxic amounts of venom, such as would happen if the animal encountered a whole nest of bees, or in the event of a snakebite," Stanford pathologist Stephen Galli, a co-author of the study, said in a statement. "Anaphylaxis probably represents the extreme end of a spectrum of IgE-associated reactivity, which in some unfortunate individuals is either poorly regulated or excessively robust, so the reaction itself can become dangerous to them."

SOURCES: Marichal­­ et al. “A Beneficial Role for Immunoglobulin E in Host Defense against Honeybee Venom”; Palm et al. “Bee Venom Phospholipase A2 Induces a Primary Type 2 Response that Is Dependent on the Receptor ST2 and Confers Protective Immunity.” Immunity published online 24 October 2013.