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Q1: I come out in a rash when I eat some brands of chocolate - the more expensive sort - but do not when I eat the best known brand on the market in Britain. Does this mean I have a food allergy or is this an intolerance? Name and address withheld.

A1: Thank you for drawing attention to the difference between a food allergy and food intolerance. A food allergy is the problem that can arise when a protein in the food acts as an allergen that stimulates the production of antibodies that lead to an abnormal response of the immune system. This abnormal response may be no more than an itchy rash but if it is more severe it can lead to overall swelling of the tissues including those of the face, mouth and throat. The allergic response may also in the very worst cases cause problems with breathing, vomiting, diarrhoea and even collapse.

Food intolerance is the condition in which a person may be unable to swallow - possibly only because of repugnance to the food's taste or smell - or because of an abnormality in their enzyme production that may prevent its normal absorption, an inflammation of the gut. Although food intolerance may also give rise to generalised symptoms as there are no changes in the antibody levels these are not true allergies and there is no abnormal immune response. The physical cause, if any, of a case of food intolerance is often undetected.

From the description of your rash it sounds as if you might have a mild allergy to one of the constituents of chocolate. As you probably know the more expensive and the darker the chocolate, the more likely it is to have a high proportion of cocoa. In the cheaper brands, including the best known, the proportion of cocoa in the chocolate is very low. Although the expensive high-level of cocoa in your case may induce a rash, in most people it is better for them. The darker the chocolate, the richer it is in the anti-oxidants that have benefits for the cardiovascular system as well as other welcome effects on a person's general health.

Q2: I became allergic to oysters about ten years ago, though I can and do eat other kinds of shellfish. I avoid eating a large amount of crab or lobster, for I get rashes if I do. Recently, rashes appear when I have had beef or sometimes pork. Does this mean that I am becoming allergic to other types of protein? Is there anything I could do, apart from not eating suspected sources? Thank you. Name and address withheld.

A2: Shellfish allergies are very common and it seems from your history that you have an allergy to shellfish. This is most obvious when you eat oysters. Once a patient has an allergy to one substance it is frequently found that the allergic tendency spreads to other substances. A few years ago the newspapers reported about the allergies suffered by a well known chef Shane Osborn. His allergy started after preparing scallops and lobsters but in time spread to other forms of fish and even mushrooms and aubergines.

Similarly once a patient becomes allergic to one type of tree pollen, berry or fruit doctors and the patient notice that he or she frequently becomes allergic to other berries and fruits. Likewise an initial allergy to one type of grass pollen, so that someone develops hay fever, rarely remains confined to that grass but spreads to a wide variety of grasses, flowers and trees.

I suspect that you are becoming allergic to other proteins as well as those in shellfish but another possible explanation is that your pork or beef has been cooked in saucepans or pans that have previously been used to cook shellfish. Only the smallest trace of the protein that is an allergen may be enough to stimulate a reaction. I would discuss your allergies with your doctor and ask if you can see an allergy specialist. The waiting list is usually a long one but if you know you have a tendency of developing an allergy it is as well to have it carefully defined. In some cases de-sensitisation is possible and techniques for this are improving year by year. No one can predict when an allergy might not suddenly become more serious. If this is considered a danger an Epi pen to give an immediate dose of adrenaline should be carried at all times. Meanwhile avoid shellfish.

Q3: I was diagnosed with a cashew nut allergy as a small child will I still be allergic? I've never tried the nut. Name and address withheld.

A3: Not necessarily but I wouldn't risk it. Nut allergy is perhaps of all the allergies the one that seems to be the cause of the most severe and dangerous of symptoms. An allergy specialist can take a careful history and can perform skin and blood tests. If the allergy is likely to be serious the patient should not only avoid the precipitating foods and the utensils used to prepare them but should steer clear of all nuts. It might also be necessary for you to carry an Epi pen. These are very simple devices that enable you, or a friend to give an immediate injection of adrenaline if serious symptoms develop.