Ah, glorious springtime. It brings flowers, warmer temperatures—and for many, incessant sneezes and sniffles. Everybody curses allergies as annoying at best, and some allergic reactions—such as anaphylaxis, which rapidly lowers blood pressure and closes the airways—can be fatal. But a handful of researchers now propose that allergies may actually have evolved to protect us. Runny noses, coughs and itchy rashes keep toxic chemicals out of our bodies, they argue, and persuade us to steer clear of dangerous environments.
Most immunologists consider allergies to be misdirected immune reactions to innocuous substances such as pollen or peanuts. Viral and bacterial infections invoke what are called "type 1" immune responses, whereas allergies involve "type 2" responses, which are thought to have evolved to protect against large parasites. Type 1 responses directly kill the pathogens and the human cells they infect; type 2 works by strengthening the body's protective barriers and promoting pest expulsion. The idea is that smaller pathogens can be offensively attacked and killed, but it's smarter to fight larger ones defensively.
But Ruslan Medzhitov, an immunobiologist at Yale University, has never accepted the idea of allergies as rogue soldiers from the body's parasite-fighting army. Parasites and the substances that trigger allergies, called allergens, "share nothing in common," he says—first, there are an almost unlimited number of allergens. Second, allergic responses can be extremely fast—on the scale of seconds—and "a response to parasites doesn't have to be that fast," he says.
In a paper published April 26 in Nature, Medzhitov and his colleagues argue that allergies are triggered by potentially dangerous substances in the environment or food to protect us. (Scientific American is part of Nature Publishing Group) As evidence, they cite research including a 2006 study published in The Journal of Clinical Investigation reporting that key cells involved in allergic responses degrade and detoxify snake and bee venom. A 2010 study published in the same journal suggests that allergic responses to tick saliva prevent the pests from attaching and feeding. This mechanism, he argues, is distinct from the classic type 2 response the body uses to defend itself against internal parasites.
More generally, hated allergic symptoms keep unhealthy environmental irritants out of the body, Medzhitov posits. "How do you defend against something you inhale that you don't want? You make mucus. You make a runny nose, you sneeze, you cough, and so forth. Or if it's on your skin, by inducing itching, you avoid it or you try to remove it by scratching it," he explains. Likewise, if you've ingested something allergenic, your body might react with vomiting. Finally, if a particular place or circumstance ramps up your allergies, you're likely to avoid it in the future. "The thing about allergies is that as soon as you stop exposure to an allergen, all the symptoms are gone," he says.
Importantly, Medzhitov notes that although allergies are intended to be helpful, they are sometimes excessive and detrimental—the body can go too far. And allergies don't always make sense. "I would say that food is still mostly innocuous," says Dale Umetsu, an immunologist at Children's Hospital Boston, yet "food allergies affect one in 12 kids." How is that protective? According to Medzhitov, foods may have proteins in them that are harmful or they might mimic potentially harmful substances. (With food, he says, there's often little consensus about what, exactly, the offending allergen is.) And one has to think of the evolutionary past, he adds: for our ancestors hundreds of thousands of years ago, many plants that looked like food were toxic, so allergies may have evolved to protect us from them. Finally, he says that some allergies may develop through a "guilt by association" mechanism: An individual might develop an egg allergy after eating eggs in a polluted environment, for instance. "This is a type of detection by proxy—you use some cue, like smell, or a visual cue or taste, to indicate if a food is associated with something that's noxious. Next time you're exposed to it, you avoid it."