About 40,000 years ago, when humans roamed Earth alongside Neanderthals and Denisovans (another now-extinct species), there was some inter-species breeding going on between these three species that influenced Homo sapiens sapiens as they exist today.
To this day, people whose family history extends beyond Africa have between 1-6% DNA overlap with Neanderthals or Denisovans. Two studies published in the American Journal of Human Genetics show that these other species in the Homo genus gifted humans with three genes related to the immune system. These genes, researcher Janet Kelso told NPR, provide humans' "very early immune response."
It's nice that humans picked up some genetic tips from Neanderthals and Denisovans on how to survive when they swapped DNA. But these shared genes also triggered a greater chance in humans of treating pollen, pet hair, and other mundane things as threatening forces. "So I suppose that some of us can then blame Neanderthals for our susceptibility to common allergies like a hay fever," Kelso said.