A method for determining the presence of delayed food allergy and intolerance against antigens extracted from modified foods. The method includes determining a level of antibodies against a modified dietary food antigen in blood and mucosal samples from the patient and comparing the level with normal levels of the antibodies. Dietary antigens that were tested include milk and modified milk products; eggs and modified egg products; meat and modified meat products; fish, mollusks, and crustaceans and their modified products; oils, fats and their modified products; grains and modified grain products; pulses, seeds kernels, nuts and their modified products; vegetables and modified vegetable products; fruits and modified fruit products; sugar, modified sugar products, modified chocolate products and confectionery; and spices and their modified forms.