When the LAND ITSELF REJECTS them! 😂
(media.scored.co)
It seems like it's the same as any pollen allergy. If you, or your ancestors, are not frequently exposed to a particular pollen compound, you're at risk for developing an allergic reaction to it. From what I've just read, olive pollen allergy is somewhat common across the Mediterranean, but Arabs in the Levant are particularly less affected than any other population, probably because they've been exposed to significant amounts of that pollen for many generations, whereas Israeli jews have not.