Not all allergies are constant and predictable. Many appear to come and go. The key to this shifting pattern is the concept of ‘cyclical’ and ‘fixed’ allergies.
Fixed allergies, as the name implies, never really change. Once acquired, they are with one for life. In general fixed allergies are severe. Allergies to insect stings, strawberries and shellfish tend to be of this type. These are usually immunollogically-induced allergies and, in accordance with our present understanding at any rate, there is no reason to expect them to alter.
Cyclical allergies, on the other hand, vary considerably in the severity of reactions they produce. The more often the allergen is encountered, the worse the reaction becomes. Conversely, if the allergen is avoided for a long period, the reaction tends to dampen down.
The actual period of avoidance varies a great deal. In some cases as little as a few days may result in loss of response to a single mild dose. Other people may have to avoid the allergen for many months. The majority of allergens lie somewhere in between.
The cyclical effect is of great importance when it comes to allergy food challenge testing. The optimum interval between avoidance and testing for a food is five to ten days. Five days are needed for unmasking (see hidden allergy from the mechanisms of allergy page) but, beyond that time, the sooner the tests are carried out the better. After ten days certain allergens may begin to lose their effect and so be missed on a single challenge feed. As a result the patient may consider a food safe, eat it frequently and suffer baffling exacerbation symptoms.
This cyclical nature of allergens means that it is not usually necessary to avoid an allergen for life or, indeed, for more than a few months at a time, before trying it again. However, the patient must understand that returning to a frequent intake of the allergen will not work – it will just make the symptoms start up all over again. A hostile food will always have to be treated with some caution.
Rotation dieting is an attempt to prevent the build-up of cyclical allergies. By eating foods in line with a careful timetable, say every four days, it is usually possible to maintain the safe character of a food. Remember also, a food eaten below its ‘threshold dose’ will appear not to cause a reaction.