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.
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