Why Did Humans Evolve to Have Allergies
Why would humans evolve to be put in the hospital by a harmless peanut?
Humanity's best guess
It turns out we don't exactly know why allergies are a part of human evolution, but some compelling theories may hold the answer.
The most popular belief is that allergies are a mistargeted immune response to protect us against parasites. Proteins responsible for allergic reactions have significant similarities to many parasite proteins, so it is proposed that the immune system incorrectly recognizes harmless allergens as harmful parasites.109 Additionally, in the modern world, we are exposed to far fewer parasites than in the past, so it has been suggested that parasites, which we have coevolved with for millions of years, may play an essential role in training and regulating our immune system's reactivity towards allergens.110
Another theory disagrees with the idea that allergies are mistargeted and proposes that allergies have evolved as an essential defense against harmful environmental substances such as venoms. Additionally, the theory suggests that allergic responses have been an important tool that signaled humans to avoid suboptimal and potentially toxic environments.111 This theory seems to have some support from a study showing that an allergic response in mice can be protective against the lethal effects of various venoms and perhaps other toxic substances.112