I worked with a woman who had an apple allergy - no-one believed her either - it's because a lot of people only know about the 'main' 12 or so. DS1 is fructose (sucrose and dairy as well) intolerant, so that includes apples. Bananas are his worst though. There was a big thing back in the early 90s (roughly then) when carrots caused a lot of reactions. At that time it was apparently related to some of the pesticides that are routinely used in their 'production'. Even washing didn't always help as this product had seeped into the veg itself. I was reading online the other day about a 4 day diet. Foods are grouped into type and you don't eat anything else from the group within the following four days. It apparently reduces the risk of allergic reactions to certain types of food as it doesn't overload the body with similar irritants.
My DS has always reacted to apple skins by throwing up violently and sometimes getting hives. I thought it was because there is sometimes milk protein in the wax on apples (and once posted on here to that effect) I know what you mean about nobody believing in apple allergy. It sounds so unlikely, doesn't it?
Finding new allergies can seem so overwhelming.... I know this probably going to sound a bit hollow and uncomforting when you are going through such a horrid time, but my DS has just outgrown his allergy to peaches and plums (related to OAS) and can also now eat tomatoes and citrus.