Illustration by EUGENE CUBILLO

Q. What causes allergy? Why do some people get allergic to certain things to which they are not previously allergic to? How can you prevent allergies?
– Miguel P., San Fernando City

A. Allergy or allergic reaction refers to an inappropriate—and sometimes overwhelming—reaction of a person’s immune system to something (called allergen) that is harmless to most other people. Allergens come in a variety of forms—pollen, house dust, animal dander, molds, cockroaches, drugs, certain foods, etc. They trigger an allergy by getting in contact with the skin or the mucous membrane of the eyes, or by getting inhaled, ingested, or injected.

Why a particular substance can elicit an allergy in some people and not in others is unknown, but it is almost certain that allergy has a genetic basis. In fact, a family history of allergy is the most important predisposing factor for the condition.

The signs and symptoms of allergy are usually mild sneezing, itchy and runny nose, watery eyes and skin itch or rashes including hives. Rarely, allergy manifests in the form of an anaphylactic shock, a life-threatening condition that is characterized by sudden swelling and constriction of the airways causing difficulty of breathing and widening of blood vessels causing a drop in blood pressure. Incidentally, among asthmatics, an allergy can trigger an asthma attack.

Allergy is one of the four forms of hypersensitivity reactions that the immune system can exhibit. The immune system is the body’s main defense system against infectious agents and harmful substances. It is composed of a variety of cells, chief of which are the white blood cells.

Allergy is otherwise known as type I or immediate hypersensitivity reaction and consistent with its technical name, its signs and symptoms usually appear 15 to 30 minutes from the time of exposure to the allergen, although sometimes they appear after a 10- to 24-hour delay.

It is not unusual for a person to develop an allergy following contact with an allergen that he/she was not allergic to previously. In fact, as a rule, an allergic reaction does not occur if a person is encountering an allergen for the first time. This initial encounter merely sensitizes the person to the allergen and produces no symptoms. It is during subsequent encounters with the allergen that an allergic reaction develops. How is this so?

A new allergen that enters the body will invariably run into B cells (a type of white blood cell). This meeting will stimulate the B cells to produce antibodies called immunoglobulin E (IgE) that attach themselves on the surface of basophils (another type of white blood cell) and mast cells (a type of connective tissue cell) where they simply wait for another meeting with the allergen that triggered their production.

During the next and subsequent times the person encounters the allergen (i.e., days, months or years after the initial meeting) to which he/she has been sensitized, the IgEs that are attached to the basophils and mast cells capture the allergen. This prompts the basophils and mast cells to release inflammatory chemicals that are responsible for the signs and symptoms of allergy and that also attract other cells such as eosinophils (another type of white blood cell) that add more inflammatory chemicals.

The best way to prevent an allergic attack is by avoiding one’s allergens. Many people know their allergens. Those who do not can often identify theirs by undergoing a test for allergens.

For people who can’t avoid their allergens, immunotherapy is sometimes employed to desensitize them from their allergens. The most common form of immunotherapy consists of injecting tiny amounts of the allergen under the skin over a period of time. Allergen immunotherapy is however not always effective.

There are also some drugs that are designed to prevent and/or relieve the signs and symptoms of allergy including antihistamines, cromolyn and corticosteroids.

(E-mail inquiries on health matters to: medical_notes@yahoo.com or wellbeing@mb.com.ph)