It is important to stress at the outset that contrary to popular belief, true food allergy is not very common. Actual figures for the number of people affected are difficult to estimate, but in general it is thought that no more than between 1 and 4% of the population suffers from a definite food allergy. Interestingly, true food allergies are usually (but not always) seen in people who also have one of the other common allergic conditions, such as hay fever or asthma. Obviously many more than 4% of the population think that they have a food allergy but several careful studies suggest that as many as half of these patients have no allergic symptoms at all. However, they are often so firmly convinced of their allergy that it is difficult for a doctor to persuade them otherwise.
So how do we pinpoint true food allergy? To begin with, we have to look carefully at the mechanisms and symptoms involved.
How do food allergies work?
Studies of food allergy and food intolerance have always been linked and until recently, most attempts to find out how they worked concentrated on the immune system. The root cause of classical or true allergies - including those to food - is the production of Immunoglobulin-E (IgE) antibodies to the offending molecule or allergen (a substance which causes an allergic reaction). IgE is an antibody which normally does the job of protecting the body against parasite infestation, but in allergic people this mechanism has gone awry. When this happens, instead of having a protective role like that of other antibodies, IgE may produce harmful effects in the body. Abnormal amounts of it are produced in response to protein substances which are completely harmless to non-allergic people. IgE binds to cells rich in chemical substances, known as 'mast' cells.
Once an allergen reacts with the IgE molecule, a complex set of biochemical reactions is set in motion and as a result potent chemicals 4such as histamine, serotonin and prostaglandins are released into the body. These are what cause the intense itching, swelling and inflammation. A more serious effect occurs when really large amounts of these chemicals are released. They enter the circulation system,causing capillaries to enlarge and become more permeable, which in turn results in a drop in blood pressure. This reaction - known as anaphylaxis - may be fatal. Laboratory tests in people who have true food allergy show high levels of IgE, but this is not the case in food intolerance. In fact, the involvement of IgE is what makes the crucial distinction between the two.
The specific area where IgE is important is in the wall of the digestive tract. When an allergic person eats an offending food it could combine with IgE in the tract and set off local inflammation, which in turn makes the wall of the tract more permeable and allows undigested food molecules to get through. Once absorbed, these molecules can cause a whole range of symptoms.
Several recent studies have shown that patients with food allergy tend to have 'leakier' digestive tracts than do healthy people, although this is not always the case. When food molecules do enter the blood they usually form what are termed 'circulating immune complexes' (CICs) with antibodies. These CICs may cause inflammation and could account for the joint pains and migraine which some patients sufferfrom.
IgE and CICs are not the only immune pathways involved in food allergy. Another possible mechanism is via lymphokines' or `cytokines', which are hormone-like substances that form a communication link between different cells in the immune system. In the case of food allergy, an abnormal immune response to food molecules could lead to the release of lymphokines, followed by unpleasant symptoms such as headache, lethargy, dizziness, joint pains and nausea. Other factors, such as the incomplete digestion of particular food proteins, or certain enzyme deficiencies, may also be relevant in allergic people.
Experts believe that at least two of the defects mentioned above, plus certain environmental triggers, are necessary for the symptoms of food allergy to appear. In other words, the theory is that a combination of several factors could be responsible. If this is so, it could explain why no two victims of food allergy seem to have the same symptoms.