Food crops are being genetically modified usually to come up with more robust -and therefore more productive and profitable- varieties or to improve its nutritional content. Take for example my previous post on tomatoes with enhanced flavonol content.
Those who oppose the use of genetically modified crops, however, are concerned that such food may cause food allergic reactions as they are made to produce snippets of “foreign protein” (GeneticsandHealth has several entries discussing the nitty-gritty of genetically modified food here, here, and here).