WHAT DOES IT MEAN TO BE ALLERGIC?

In 1902, two French scientists injected dogs with a small amount of extract from the sea anemone.  Nothing happened.  A week later, they repeated the procedure in exactly the same way – and watched, amazed, as the animals developed severe reactions.

The dogs had somehow become sensitive to the formerly harmless substance. The researchers had discovered allergy.

In the decades since that landmark series of experiments, scientists have come to know a great deal about the phenomenon called allergy, which afflicts millions of people around the world.  Researchers have found that, ironically, allergy results from a malfunction of the immune system, a collection of cells and molecules that exists to protect us from harm.

Normally, the body learns to defend itself through experience – by encountering, battling and remembering one enemy after another.  For decades, medical science has taken advantage of this process by using vaccination to create immunity.  Vaccination “teaches” the immune system to “remember” a disease.

Allergic reactions occur when the immune system mistakenly identifies innocent foreign substances as potentially harmful.  Such substances are called allergens.  Pollens, mold spores, house dust mites, animal dander and foods are among the allergens that most frequently cause allergic symptoms.
Not every adverse reaction to something in the environment is an allergy.  To qualify as an allergy, a reaction must be triggered by one or more specific things –be it ragweed or rutabaga- and the immune system must be involved in causing the reaction.  Problems caused by toxins, irritants, or intolerance don’t meet this definition, because the immune system is not involved.

Antibodies are a part of the immune system that play a central role in allergy.  These protein molecules act as “scouts” that identify foreign invaders so the body’s powerful weapons of defense can be deployed with precision and speed.  Antibodies are produced by a class of white blood cells known as B cells, which in turn are regulated by another key group of blood cells called T cells.

In 1965, scientists at the National Jewish Center for Immunology and Respiratory Medicine discovered a particular class of antibody called immunoglobulin E (IgE).  This molecule plays a key role in the most common type of allergic reaction.

The illustrations on page 13 demonstrate how this kind of allergy can develop.  Each fall, over a field of ragweed floats an invisible cloud of pollen grains, which are carried by the wind into a nearby town.  The pollen is inhaled by a child who may have never been exposed to this substance before. 

Because of some predisposing genetic factor, the child’s immune system produces large numbers of IgE antibodies, all designed to respond specifically to ragweed pollen.  Several of the antibodies attach themselves to cells in the child’s nasal passages and upper respiratory tract.  These cells, known as mast cells, contain strong chemicals called mediators, the best-known of which is histamine.

Later, when the child inhales ragweed pollen again, proteins from the pollen bind in a lock-and-key fashion to these antibodies on the surface of the mast cells.  This sets off an explosion of sorts, and the mediators burst from the mast cells, causing blood vessels in the nasal passage to swell and leak fluid.

The child in the foregoing example has developed “hay fever” – which involves neither hay nor fever.  Doctors prefer to call it allergic rhinitis.  The end result is symptoms – the sneezing, sniffling, stuffy head and red, watery eyes that are hallmarks of this common allergic condition.

Not all allergic reactions happen in the same way as the pollen allergy in this illustration.  Some allergy attacks occur immediately in a previously sensitized individual after exposure to an offending substance, while others take hours or days.  Scientists theorize that the biological events leading up to delayed reactions are different from those that result in immediate allergic responses.

The symptoms of allergy aren’t limited to the nose and eyes.  Other organs can be affected – especially the skin, the lungs, the digestive tract and the blood vessels.  Fatigue frequently accompanies chronic allergy symptom.

The skin conditions most often associated with allergy are urticaria and eczema.  Allergic urticaria –sometimes called hives- is the appearance of itchy red swollen areas on the skin, usually soon after exposure to an allergen.  The hives may be small bumps or large irregularly shaped areas.  Another type of hives is physical urticaria, when hives develop in response to physical stimuli such as cold, pressure, or a hot shower.

Eczema, also known as atopic dermititis, is a dry, scaly skin rash that is associated with intense itching.  The skin may crack and bleed, or become infected.  Symptoms may last for months at a time, clear up, and then return.

Not all asthmatics have allergies, but 70-80% of children with asthma have a significant allergic component as a complicating factor to their asthma.

Allergy can cause abdominal cramps, nausea, vomiting or diarrhea.  These symptoms are most common with food allergy, but can occur with other types of allergy as well.

Generalized anaphylaxis is the most serious kind of allergic reaction.  Fortunately, it’s rare.  Only a few types of allergens such as venom from stinging insects, drugs, and (rarely) foods cause anaphylaxis.  Symptoms usually appear rapidly – within seconds or minutes – after exposure to an allergen, but in a few cases, reactions have been delayed as much as 12 hours.

In anaphylaxis, cells in the immune system release large amounts of chemicals – including histamine.  As a result, blood vessels dilate and begin to leak fluid into surrounding tissues, producing swelling.  Symptoms may include hives, itching, light-headedness, vomiting, diarrhea and severe breathing difficulty.