The most common foods people are allergic to are: milk, egg, peanut, tree nuts, fish, shellfish, soy, and wheat. It doesn't really mean that the entire food is the allergen, but rather some ingredient of it. And often they aren't allergies at all, but various food sensitivities.
Allergy is a general term mistakenly used to include pseudoallergies and food sensitivities, which are not really allergies, since they don't involve the immune system and antibodies. Pseudoallergies are not caused by the body's immune responses. Food sensitivities are caused by enzyme defects preventing proper digestion of certain food ingredients. A lot of lactose intolerance is actually an enzyme defect and not an allergy.
As far as real allergies, it is somewhat of a mystery of why the antibodies would attack it's own body. The Ayurveda believes that allergies are caused by the improper digestion and poor elimination (not pooping enough) that creates toxins that aggravate the immune system to the point of the antibodies becoming over-zealous. They think that improperly digested foods are passed along to their destination organs and cells when the body is thus overwhelmed by food. The scientists may think it's a mystery why the antibody would attack an innocent cell. Here is my opinion on that: Say an improperly digested food particle which is too big is passed on for a cell to eat, and it is shaped like a star. A cell is soft and round, and when it swallows the huge chunk of food it stretches itself over the star shaped particle and thus acquires pointy corners. An antibody comes along, sees the stuffed cell with pointy corners and thinks it is the offending food and kills it.
One of the best traditional methods of finding out what you're allergic to is fasting until all the food is out of your system, cleaning out your stomach and the intestines by drinking lots of pure water from natural springs that comes in glass bottles, not plastic, and does not contain fluoride or chlorine or any additives, and you might want to do an enema to clean the colon as well. After you fast for a day or two you should notice any original discomfort, like an arthritic pain, to vanish or diminish noticeably. If fasting is too hard on you, you can probably get away with eating just one safe food for 4 days. If after 4 days you do not notice any dramatic improvement in your symptoms, try a different safe food for the next 4 days. Once your symptoms vanish or improve dramatically, you can re-introduce foods one at a time, using only the pure organic ones, and starting from the list of 'safe' foods. I have not been able to find a really good safe foods list, because there so many different allergies. The safest foods are reported to be: lamb, pears, apples, rice, most vegetables, peas, beans, zucchini, and the gluten-free nut-free chemical-free grains, such as teff or quinoa. (By the way, I know of a local grocery that sells teff real cheap, here's the link: Ethiopian Grocery But you can't be certain with any food. For instance, cucumbers are considered safe, but I have a friend who becomes violently ill if she eats cucumbers. I myself get a horrible case of painful tongue if I eat raw carrots or pineapple. So watch you bodily responses diligently and do not take any shortcuts. So you start out by eating only one safe food after your fast and wait 4 days to see if any adverse reaction occurs, and if it does, stop eating it and add it to your personal food sensitivities list, then add another food, and so on. If you cook any of these foods, do not fry them, boil them or steam them using pure filtered or spring water. If you like your veggies stir fried, then take 4 days just for trying each type of a vegetable stir fried in organic oil, and keep the same type of oil for 4 days. When you get to questionable foods, like eggs, cheese and stuff, make sure they are organic, from free-range chickens and cows. The meat and milk should come from cruelty-free organic farms that don't use any hormones or questionable foods. The reason for that is that there maybe some bad energy or harmful to us chemical or hormonal changes in the bodies and eggs of mistreated or sick animals. Don't forget to wait 4 days at least to add the next food. You might have to spend more on your food, but it is totally worth it, since it will greatly reduce or eliminate your doctor's bill. Besides you'll eat less, which is very healthy in itself, lose a bunch of weight and look like a million dollars.
An Indian scientist that moved to the US and did a lot of research on causes of allergies expressed his opinion that insufficient drinking of clean water is what is causing them. At least that is a reasonable one cause that can be easily eliminated by following the popular advice and drinking a glass of clean water every couple of hours. What joy if that was your only problem!
Here is a good water filter to get, it gets rid of chlorine and fluoride as well as the other contaminants. Britta filters you buy at the local drugstore do not eliminate fluoride. Filtering your own water maybe a better way to go than buying a bottled water, since they sell it in plastic bottles, and plastic bottles have been recognized to leak a dangerous carcinogen into the water.
An ancient folk method of curing allergies is drinking your urine. This was done successfully throughout the centuries. You don't have to drink a full glass of urine, that would be gross, just a couple of drops dissolved in water or juice would suffice. The reason that works is that your urine contains the histamines and the information on what anti-histamines your body needs to manufacture exactly, it is really kind of like getting an allergy vaccine that's custom tailored to your body's specific needs.