During food allergy, the immune system usually reacts to a fragment of a protein (not carbohydrates or fats). The reacting protein fragments in the blood have a distinct shape. In other words, the immune system identifies this protein fragment as an antigen because of its shape. An antigen that is part of a food molecule is called an allergen.
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An antigen is any substance that causes the immune system to make antibodies. If the antigen causes an allergic reaction, it is called an allergen. The antigens on the surface of bacteria do not cause an allergic reaction and so are not called allergens.