Grass pollen is related to substances found in melons, tomatoes, oranges, peaches and celery. And the pollen from alder trees might cross-react with apples, cherries, peaches, pears, celery, parsley, almonds and hazelnuts.
Not everyone with pollen allergies develops this hypersensitivity. Roughly 25 percent of people with a significant pollen allergy -- meaning their symptoms are sufficient enough to require medication to be comfortable during allergy season -- may experience oral allergy syndrome, Costa said.
And yet, a good majority of those with hay fever, or allergic rhinitis, as physicians tend to call it, are often unaware that this reaction to foods has any connection to their seasonal allergies.
Although the itchy mouth and throat can occur any time of year, the reaction might be more common or severe in spring and fall when there's more pollen in the air.
It's a form of contact hives in your mouth, explained Dr. Anna Nowak-Wegrzyn, a pediatric allergist at Mount Sinai School of Medicine in New York City. The reaction occurs when the food is in contact with your mouth, and it doesn't progress beyond that point.
Unlike food allergy symptoms, which tend to first be seen in infants and young children, food-pollen allergies typically occur in children who are a little older and in adults.
Allergist Dr. Harold Nelson, a professor of medicine at National Jewish Health in Denver, described oral allergy syndrome as a "localized allergy phenomenon," with its main symptoms in the mouth and throat. The plant proteins that trigger the allergic reaction don't survive the passage through the digestive system and get destroyed by stomach acids and digestive enzymes, he said.
In a food-pollen allergy, the primary problem is the pollen. In a classic food allergy -- say, to peanuts, or shellfish -- the main culprit is a protein found in the food itself.
If this foreign protein, known as an antigen, passes through the digestive system and gets absorbed, explained Nelson, the immune system reaction is a systemic or body-wide effect. That's why you may see symptoms, such as cramping, hives, diarrhea, and pain with a true food allergy.
The main question Nowak-Wegrzyn asks a patient who gets this cross reaction during pollen season is, "How uncomfortable does this make you feel?"
For some people, removing the peel from the fresh fruit or vegetable will do the trick. And in other cases, the freshly picked versions of produce may be better tolerated because storing the food increases the amount of allergen.
From time to time, experimenting with different varieties of the offending food -- say, an apple -- might not cause symptoms. Nowak-Wegrzyn doesn't tell people with food-pollen allergy syndrome to strictly avoid the food, but to trying it out of season or processing it differently.
She said getting allergy shots for hay fever seems to help oral allergy syndrome. Rinsing the mouth with water after eating the offending food may also dilute the protein on the mouth's surface.
As a pediatric allergist, Nowak-Wegrzyn occasionally finds herself reassuring parents that a child complaining of pollen-food allergy symptoms is not looking for a convenient excuse to skip the healthy fruits and vegetables.
"Sometimes, when I tell parents that the child is not making this up, they are relieved to know that this is real," she said.