Medical Questions » Allergy Questions » Question No. 21
| Question: | Just what is an allergy? How do they occur? I have been able to eat oysters all my life, but now I get an itchy rash if I even touch one. How can this happen? |
| Answer: | An allergy is an excessive reaction to a substance which in most people causes no reaction. An allergy can occur to almost any substance in our environment. It may be triggered by foods, pollens, dusts, plants, animals, feathers, furs, mould, drugs, natural or artificial chemicals, insect bites and gases. No-one is totally immune to all possible allergic substances, but some individuals are far more susceptible to a wide range of substances than other people. This tendency towards allergies tends to run through a family, but the form the allergy takes (eg. hay fever or rash) and the substances that the person reacts to (eg. oysters and bee stings) can vary from one generation to the next.
Allergy reactions may be very localised (eg. at the site of an insect bite, or in just one eye), may occur suddenly or gradually, may last for a few minutes or a few months. They may involve internal organs (eg. lungs), or be limited to the body surface (eg. skin or nose lining). Allergies that cause significant discomfort or distress to the patient occur in 10% of the population.
An allergy differs subtly from a hypersensitivity reaction, where a normal reaction, experienced by most people, is exaggerated in some individuals. Often the difference is of little consequence to the sufferer and may only be determined by measuring specific substances (called immunoglobulins) in the blood. Side effects of drugs, psychological reactions, personal dislikes and the results of over-exposure to substances, must all be differentiated from a true allergy.
When a person is exposed to a substance to which s/he is allergic, the body reacts by releasing excessive amounts of a substance called histamine from mast cells that are found in the lining of every body cavity, and in the skin. Histamine is required at times to fight invading substances, but when released in excess, it causes inflammation, redness, and swelling, and in body cavities, the excess secretion of fluid from the involved surface. Intense itching often also occurs.
It is rare for an allergy to occur on the first occasion that a person comes into contact with a substance or food (in your case, oysters), but an allergy reaction can occur on the second, third or 300th time that the substance is encountered. Developing an allergy after years of exposure is therefore, not unusual. |
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