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In theory, any substance can be an allergen. In practice, some substances are inherently more ‘allergenic’ (likely to cause allergy) than others and consistently cause more problems. The antibodies of the immune system need a physical handle to grasp on to, and certain chemical structures provide this better than others – proteins, in particular, are more allergenic than other types of molecule. Wool, for instance, is a protein, as are pollens, and these cause allergies more readily than molecules such as cotton, which is not a protein. In the case of foods, proteins are more likely to cause trouble than, say, fats or oils.
A molecule has to be above a certain size for the immune system to react to it. Chemicals are too small to trigger the immune system by themselves, but they can combine with other, larger molecules and form ‘haptens’, which can then trigger the system.
Anything that can be absorbed into the bloodstream can be an allergen. This means anything inhaled, swallowed, injected or absorbed through the skin or mucosa. It was commonly believed that the place where your symptoms occurred was the site of initial sensitisation. This is not now thought to be the case, since symptoms such as asthma can, in some individuals, be shown to result from substances that have been swallowed rather than inhaled. Allergens are carried by the bloodstream until they meet the place where the mast cells are located, and it is there that the reaction occurs. The most common allergens are shown in Table 1 (see Part 4 for further details). In this book, the word ‘allergen’ is used wherever possible to mean a substance that causes true allergy. If a food or other substance causes intolerance, or some other sensitivity reaction, they will be called ‘substances that cause reactions or sensitivity’.
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