Curiosities: Why do people sometimes develop late in life an allergy that never bothered them before?

Most allergies — especially to airborne allergens associated with runny noses and itchy eyes — come on in the teenage years or early twenties, according to Mark Moss, a professor of pediatrics and immunology.

Late allergy emergence is a bit of a mystery, Moss said, so much so that late-developing allergies aren’t seen as late-developing at all. If you have the potential for an allergic reaction, it’s probably always been there, waiting for the right mote of dust to waft by.

Developing an allergy takes an initial “sensitizing” exposure when your body figures out it doesn’t like something. Putting off that step is the most likely cause for putting off a reaction. “Someone who is developing allergies later in life may not have been exposed to the right allergen yet, just might not be sensitized yet,” Moss said.

But there are cases in which late exposure just doesn’t seem likely.

“We see people who grow up in the Midwest and are definitely exposed to ragweed, but they don’t develop a problem allergy until they are 30 or older,” Moss said.

In that case, it’s possible that a new set of sneezing fits was caused by a coattail-riding allergen. Your typical reaction to clouds of spring pollen may be so slight as to go unnoticed — unless you are already suffering after a visit to a cat-fancying relative.

“If you’re exposed to allergens in sequence, that first one may prime you for a reaction to the second,” Moss said. “You’re thinking, ‘I’ve never had this allergy,’ but you really just haven’t been hit that hard.”