This assumes that the affected person makes IgE antibodies to the synthetic chemical concerned (see p25), or responds with some other inappropriate and damaging immune reaction. Since the chemicals concerned are too small to act as antigens in their own right, they would have to combine with body proteins and act as haptens.
It has been suggested that this can happen with some food additives, particularly preservatives and synthetic colours. These cause chronic urticaria (nettle-rash) in some people, and urticaria is sometimes due to an allergic reaction. There are also isolated cases of tartrazine (a synthetic colouring) causing acute asthma attacks, or a severe reaction that resembles anaphylactic shock in some very susceptible people. Other food colourings, particularly the synthetic ones, have been known to cause allergic dermatitis, mainly in food workers exposed to large amounts. The preservative, sorbic acid, has occasionally caused allergic dermatitis when used in medicinal creams.
In most of these cases, the tests to show that the reaction really is an allergic one have not been carried out. And when a group of patients who were apparently allergic to tartrazine were tested for IgE antibodies, none were found. So it looks as if these are not allergic reactions at all, even though they produce allergy-like symptoms. Doctors suspect that tartrazine produces symptoms in these people by directly affecting the immune response in some way – perhaps by stopping the synthesis of immune regulators called prostaglandins, or by triggering mast cells directly. In the case of synthetic chemicals apparently causing asthma, the effect may be due to irritation rather than an allergic reaction. This is well known for metabisulphites and sulphur dioxide.
These are cases where the symptoms provoked by chemicals at least looked like allergic symptoms. In the majority of chemical-sensitive people, the symptoms are not those commonly associated with allergy. So it seems unlikely that chemical sensitivity is allergic in origin. It is possible, however, that synthetic chemicals might affect the immune response in some way. This has indeed been shown for some chemicals, but the usual effect is to lower resistance to disease, rather than to make allergies more likely.
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