Adverse reactions to food may be due to immunologic or non-immunologic causes. Immunologic conditions include antibody-mediated reactions to foods. This happens because the food contains certain epitopes that closely resemble antigens on other microbes or biologic matter, including spores and pollen.
Adverse reactions to non-toxic foods that are caused by non-immunologic mechanisms are called food intolerances. When they are immunologically mediated, they are food allergies. Food intolerances make up the majority of food sensitivity reactions. True food allergies are much rarer, comprising only 2 – 5% of all food-induced reactions.
Both food intolerance and food allergies produce a wide spectrum of symptoms. These may include cutaneous, vascular, gastrointestinal and respiratory symptoms.
In food allergies, cutaneous symptoms are the most common reactions. Isolated respiratory symptoms are never seen in a food allergy. However, anaphylactic reactions are seen only with food allergies, and may be life-threatening.
Food allergies are highly specific and occur only in response to particular food components. A minute amount of the allergen is sufficient to set off a reaction. In contrast, food intolerances are non-specific and require a threshold amount of the food to produce symptoms.
Food allergies begin to produce symptoms within minutes of ingestion of the offending food. Immunoglobulin E (IgE)-mediated reactions are characterized by their beginning within 2 hours at most from the time of ingestion. Food intolerances are characterized by a delay of several hours between the time of eating the food and the onset of symptoms, which often last for days afterwards.
The history of a person with true food allergy will often be positive for an atopic disorder, such as asthma, allergic rhinitis or eczema. Eczema occurring within the first 12 months of life is often linked to certain food allergies and there may be a family history of food allergy. Food intolerances are not linked to a history of allergies or atopy.
Diagnostic tests for food allergy include skin prick testing and IgE measurements for antibodies to specific foods. The history is important in suggesting which foods are to be tested as most allergy tests are sensitive rather than specific. In addition, the history helps to find out whether the IgE levels truly reflect a state of allergy. In food intolerance, specific serum IgE is not found to be raised.
Finally, these tests cannot help to distinguish between sensitization to an allergen in food, in the form of IgE antibodies circulating in the blood, and clinical allergic states, in which IgE is bound to mast cells resulting in mediator release from the mast cell granules. For this reason, an oral food challenge is often crucial to bring out the presence of the food allergy rather than just circulating antibodies to a particular food. This test is important when the history and the IgE level suggests that allergy to a food is possible, but it is not certain. It may be omitted when the level of possibility is high.
On the other hand, food intolerances cannot be diagnosed by immunologic skin prick or IgE measurements. Instead, elimination tests may be useful in delineating the role of a suspected food in producing symptoms. The food elimination test should be under the supervision of a dietitian and confirmation is provided by reintroducing the food in small amounts after a period of time.
In short, food allergy is characterized by the immediate onset of symptoms following exposure to an allergen in food, with acute symptoms. Anaphylactic reactions may occur. On the other hand, food intolerance symptoms are delayed and prolonged, and are never anaphylactic reactions. Serum IgE levels are raised with reference to the allergenic foods in food allergy, but never in food intolerance.