What is an allergy?
An allergy is a specific reaction of the body’s immune system to a normally harmless substance, one that does not bother most people. People who have allergies often are sensitive to more than one substance. Types of allergens that cause allergic reactions include:
- Dust mites
- Mold spores
- Latex rubber
- Insect venom
Why are some people allergic?
Scientists think that some people inherit a tendency to be allergic from one or both parents. This means they are more likely to have allergies. They probably, however, do not inherit a tendency to be allergic to any specific allergen. Children are more likely to develop allergies if one or both parents have allergies. In addition, exposure to allergens at times when the body’s defenses are lowered or weakened, such as after a viral infection or during pregnancy, seems to contribute to developing allergies.
What is an allergic reaction?
Normally, the immune system functions as the body’s defense against invading germs such as bacteria and viruses. In most allergic reactions, however, the immune system is responding to a false alarm. When an allergic person first comes into contact with an allergen, the immune system treats the allergen as an invader and gets ready to attack.
The immune system does this by generating large amounts of a type of antibody called immunoglobulin E, or IgE. Each IgE antibody is specific for one particular substance. In the case of pollen allergy, each antibody is specific for one type of pollen. For example, the immune system may produce one type of antibody to react against oak pollen and another against ragweed pollen.
The IgE molecules are special because IgE is the only type of antibody that attaches tightly to the body’s mast cells, which are tissue cells, and to basophils, which are a type of white blood cells. When the allergen next encounters its specific IgE, it attaches to the antibody like a key fitting into a lock. This action signals the cell to which the IgE is attached to release (and, in some cases, to produce) powerful chemicals like histamine, which cause inflammation. These chemicals act on tissues in various parts of the body, such as the respiratory system, and cause the symptoms of allergy.
The signs and symptoms of airborne allergies are familiar to many.
- Sneezing, often with a runny or clogged nose
- Coughing and postnasal drip
- Itching eyes, nose, and throat
- Watering eyes
- "Allergic shiners" (dark circles under the eyes caused by increased blood flow near the sinuses)
- "Allergic salute" (in a child, persistent upward rubbing of the nose that causes a crease mark on the nose)
In people who are not allergic, the mucus in the nasal passages simply moves foreign particles to the throat, where they are swallowed or coughed out. But something different happens in a person who is sensitive to airborne allergens.
As soon as the allergen lands on the lining inside the nose In sensitive people, a chain reaction occurs that leads the mast cells in these tissues to release histamine and other chemicals. The powerful chemicals contact certain cells that line some small blood vessels in the nose. This allows fluids to escape, which causes the nasal passages to swell—resulting in nasal congestion. Histamine also can cause sneezing, itching, irritation, and excess mucus production, which can result in allergic rhinitis.
Other chemicals released by mast cells, including cytokines and leukotrienes, also contribute to allergic symptoms.
Some people with allergy also have asthma, which can be a very serious condition. The symptoms of asthma include:
- Shortness of breath
The shortness of breath is due to a narrowing of the airways in the lungs and to excess mucus production and inflammation. Asthma can be disabling and sometimes fatal. If wheezing and shortness of breath accompany allergy symptoms, it is a signal that the airways also have become involved.