The hygiene hypothesis proposes that our immune system overreacts to harmless substances because there aren’t enough harmful ones around to train it to discriminate. But the question remains of why some things (pollen, a bee sting) are much more likely than others (peas, a mosquito bite) to cause a severe allergic reaction.

One theory is that allergens have certain proteins that resemble those found on parasitic worms (ironically, the very same type of creature that can be used to treat allergies.) When our immune system recognizes a parasitic worm, it produces the same type of antibodies that respond to allergens—antibodies which are actually fairly unusual. 

However, a competing theory suggests that these “harmless” substances actually aren’t: that we respond to allergens not because they remind our immune systems of parasitic worms, but because they actually physically damage our cells, shredding them to pieces. In this model, allergies are the first line of defense against toxic molecules, which our ancestors likely encountered in their environment much more often than we do today. 

For a great discussion of these two theories, see this article by science writer Carl Zimmer.

For general information on allergies, you can check out the CDC website (which also has some statistics), the NIH’s National Library of Medicine, and the Johns Hopkins Health Library.

For more on the possible genetic origins of allergies, see here. You can also listen to this podcast to hear an interview with a researcher at the Johns Hopkins School of Medicine about the hygiene hypothesis.