Millions of people suffer from hives or shortness of breath when they encounter everyday exposures such as pollens or peanuts. In their most favorable light you could think of your allergies as a really annoying super power, with telltale wheezing signaling your body senses the presence of something that you don’t see or consciously smell. Despite decades of inquiry, however, scientists remain unable to pin down why allergies occur.
Because allergic reactions basically mirror the way our body responds to parasites such as worms, working to expel them through sneezes, vomiting or watery eyes, the prevailing belief among allergy experts is that allergies are just an unfortunate misdirected immune response. A pair of new studies, however, takes a fresh look at why allergies occur and provides the first evidence that those bodily responses may be no accident at all. Rather, they could be the body’s way of protecting us against toxins in the environment.
This is not the first time the idea has been proposed, but these new works independently provide the first hard data to support it. By simulating honeybee stings and snakebites in mice, researchers found that exposure to these venoms can trigger a protective immune response in which the body creates specific antibodies to help neutralize the substances in future encounters. One study found that mice receiving a small dose of these venoms followed by a would-be fatal dose three weeks later had much higher survival rates than those given only the large dose. The researchers found evidence that mice receiving a small initial venom dose, akin to stings or bites, developed allergen-specific antibodies, which bind to cells throughout the body, priming them to quickly react to venoms. The papers, from researchers at the medical schools at Stanford University and Yale University are published in the November 14 issue of Immunity.
Knowing more about why venom allergies exist and tracing the molecular pathway of the immune response it elicits could have implications for understanding allergies to other things, too, the authors say. Itching, coughing or vomiting as a result of exposure to environmental irritants could signal that the body is ramping up a response to help you survive these substances in the future—or to predispose you to avoid them.
The studies fall short of settling the question once and for all of why we have allergies, however. For one, they do not provide any answers about why the immune system sometimes fatally overreacts with hypersensitive responses such as anaphylaxis, a potentially life-threatening allergic reaction that obstructs the airways and sparks a sudden drop in blood pressure. One theory, the authors posit, is these strong reactions are merely an evolutionary holdover: Anaphylaxis could just be the protective mechanism going into overdrive in a way that would have been worthwhile for our ancestors if the only other option was no protection for anyone against these toxins. The same principle may be at work with allergies as with sickle-cell anemia, says Stephen Galli, a pathologist from the Stanford team who focuses on immunology. With sickle-cell anemia, if you have two copies of the defective gene, you have a very serious disease but carrying just one copy helps protect individuals against malaria.
Generally, our immune systems have two modes for dealing with foreign substances. A type 1 response would kill an invader whereas a type 2 response would just expel it from the body. Pathogens such as bacteria and viruses, as well as infected human cells, trigger type 1, killing them. Parasites and other large external threats elicit a type 2 reaction—the expulsion strategy. Using a type 1 approach for something like allergens “would be like using a nuclear bomb to deal with street crime,” says Ruslan Medzhitov, an immunobiologist at Yale who co-authored one of the papers. Because pollen and venom are not parasites, many allergists have supported the idea that the immune system’s response to allergies is merely a glitch. This pair of studies, however, provides the first data suggesting why that response may be a deliberate action.