Almost any food can contain an allergen, a substance that causes an allergic reaction for some people, but doesn't affect most other people. Most food allergies come from eating a few common foods, principally peanuts, tree nuts, shellfish, eggs, wheat, corn and soybeans. An allergen doesn't have to come from something you eat. It might be something you touch or something you breathe, like pollen or even dust. Some allergens affect only a few people.
If you are one of the unfortunate people who is allergic to something you have eaten (or touched or breathed) it is important to discover which substance caused the problem. It is relatively easy to test a suspected allergen using a skin patch. If the allergen is not on the usual list of substances, discovering what caused the reaction can be quite hard.
Some proteins are allergenic. If a protein causes an allergic reaction, it is reasonably likely that a different protein with a similar amino acid sequence will also cause a similar reaction in the same person. Conversely, a protein whose amino acid sequence is not similar to any of the known allergens is not likely to be an allergen.