LATE-PHASE ALLERGIC REACTIONS
If an allergic patient is given a skin-prick test there will be a strong response -known as a wheal-and-flare reaction – almost immediately. This redness and itching subsides after some time, but then a different sort of reaction can set in, producing a larger, less itchy, but more painful lump. This is known as the late-phase reaction, and it is produced by the messenger substances called prostaglandins (see p28).
When a person encounters allergens in everyday life, late-phase reactions are more difficult to discern, especially if there is frequent exposure to the allergen. But such a
reaction can sometimes be observed in asthmatic patients, for example. Brief exposure to their allergen will produce an acute reaction almost immediately, followed by recovery, followed by a more insidious return of the asthma between four and 12 hours after the exposure. The late-phase reaction has usually exhausted itself by the next day.
Late-phase reactions are important, because they probably contribute substantially to the development of ‘chronic’ allergic reactions a long-term condition in which the patient is scarcely ever free of symptoms, although the severity of the symptoms may fluctuate. During late-phase reactions, the affected organ (eg the bronchi or the skin) tends to be more sensitive to non-specific irritants, so the symptoms may be sparked off again very easily, even if the allergen has been removed. A succession of late-phase reactions can easily lead to a situation where the organ is constantly over-reacting to minor irritants.
Certain drugs block late-phase reactions by preventing cell membranes from releasing the phospholipid molecules that would normally be used to make prostaglandins. The drugs that do this are
corticosteroids {eg prednisolone) which mimic the action of hormones produced by the body. The fact that corticosteroids are so useful in controlling asthmatic symptoms shows what an important role late-phase reactions can have in allergic illness.
Corticosteroids are not simply used for allergy treatment – they suppress inflammation generally because prostaglandins are widespread messengers, produced in a variety of ways. They therefore find a use in diseases such as rheumatoid arthritis where inflammation needs to be controlled.
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Tags: Allergies