Seasonal allergy sufferers should probably blame parasites - not grasses, weeds and trees - for their sinus woes.
It is thought pollen enzymes confuse the immune system into triggering the runny nose and watery eyes that typically occur during an allergic response. But research now suggests humans might have initially developed that defence to fight off parasites.
"Protease allergens (protein-splitting enzymes) cleave the same sensor proteins that evolved to detect proteases produced by the parasitic worms," says Dr Ruslan Medzhitov, an immunobiologist at Yale University.
"Thus the same response is induced unintentionally, and this may explain why this class of allergens has allergenic activity."
Understanding exactly how allergies work has eluded scientists, since the immune system reacts differently to allergens than other foreign substances.
Only recently have Medzhitov and colleagues identified a type of white blood cells called basophils as being responsible for many allergic responses.
For instance, when the immune system mistakenly tags ragweed pollen as a parasitic invader, basophils alert the immune system to secrete the specialised antibody immunoglobulin E (IgE).
That triggers the release of histamines, or chemicals released by cells of the immune system during the inflammatory response. That, in turn, causes those annoying symptoms of allergies.
"The point about allergies is that you respond with a class of antibodies called IgE whose function remains unknown," says Dr Marshall Plaut, chief of the allergic mechanisms section at the National Institutes of Allergy and Infectious Diseases.
It takes repeated allergen exposure over time to build up a strong IgE immune response and reduced symptoms, which is why seasonal allergies can seemingly strike out of nowhere.
It's trickier to explain why around 40% of the population is allergy prone and the rest isn't.
"The primary reason is probably genetic, and I say 'probably' because we don't have absolute evidence for it," says Plaut, adding that scientists haven't identified the specific genes responsible for allergic diseases.
Children may therefore inherit seasonal allergies from their parents, but it doesn't all boil down to DNA.
"You can't really separate genetics and environment the same people way used to do 30 or 40 years ago because we know they're very interactive, and we know that certain environmental factors can influence genetic factors," says Plaut.
Early childhood exposure to pets, pollutants and specific allergens are among the major allergy co-factors.
Though allergies might seem like nothing to sneeze about, scientists still have many questions to answer about why the immune system hiccup happens and how to prevent it.
"This is one of the big puzzles in immunology, and the answer isn't completely known," says Medzhitov.