Food Pollen Syndrome, also called Oral Allergy Syndrome, is a common problem among people with pollen allergies. It stems from a cross-reaction some people might have when the pollen they're normally allergic to are similar to the proteins in some produce. About a third of pollen allergy patients are likely affected.
Common symptoms are watery eyes and an itchy throat, mouth, or tongue. The absence of issues in nose or chest is the common delineation between pollen allergy and Oral Allergy Syndrome. Like other allergies, it's an over-reaction of the immune system to perceived threats from otherwise benign sources. In this case, proteins contained in some fruits and vegetables.
In most cases, the similarity between the proteins in the pollen produced by the plant the fruit comes from and the proteins in the plants themselves are what are to blame. It's for this reason that airborne pollen allergies and Oral Allergy Syndrome are so closely linked. Quite often, the oral reaction is eliminated when the food is cooked.
The American College of Allergy, Asthma and Immunology has more information on Oral Allergy Syndrome.