- Mast cells
- The Action Of A Reaction
- Who’s At Risk?
- How Allergies Develop
- Risk Factors
Rarely is a medical term used as frequently and casually as “allergy.” It pops up in everyday conversation to describe what we don’t like - school, work or just about anything else. But medically speaking, allergy has a very specific meaning: it is a biologic reaction to something your body doesn’t like.
Allergies are an abnormal sensitivity to allergens, which can be ingested, inhaled or touched, and which most other people can tolerate without trouble. Usually, the immune system does not waste its time by reacting against nontoxic substances in our environment, using its various weapons to instead fight the viruses, bacteria, fungi and other parasites that threaten health.
One set of weapons in your immune system (including IgE antibodies, mast cells, eosinophils, and type 2 helper T cells) is your body’s branch of defense against worms, parasite larvae, and certain fungi. In some areas of the world, we couldn’t live in full health without this function of our immune system. But in people with allergies, this set of weapons launches attacks against a perfectly harmless substance, such as ragweed pollen, animal dander or certain foods or drugs. The word allergy comes from the Greek allos, meaning other. It was first used in 1906 to refer to an altered reaction in the body’s immune system.
Mild allergies are more of a nuisance than a threat. They can requiring your vigilance so that you avoid exposure, or they may require medical treatment. Occasionally, allergies can be life threatening.
The Action Of A Reaction
Most people know their allergies by the reaction they cause - the sneezing, wheezing and other effects that are none too pleasing. What they may not know is that what happens to their body is a result of what’s happening inside it.
An allergic reaction is what happens when your immune system tries to mount a defense against what it believes is a hostile invader. During a period of sensitization, the time when you are exposed to a specific allergen, white blood cells that come in contact with particles of allergen produce proteins called IgE antibodies. If you were fighting an invasion by a parasite, this would be the first step in a cascade of events that could lead to the parasite’s death. In the case of a brief exposure to an allergen, your IgE antibodies may not trigger a full reaction when they are first formed. In either case, your immune system will be “practiced” at forming these IgE proteins the next time that the same allergy trigger comes along, so a large number of antibodies will be formed quickly.
The body produces five different classes of antibodies, or immunoglobulins - IgA, IgM, IgG, IgD, and IgE - and each class serves a different function. Those formed in response to allergens belong to the IgE class, which normally function to attack parasitic worms.
IgE antibodies bind to the allergen, such as pollen molecules, as well as two types of defensive cells: mast cells (found in the nose, skin, lungs and gastrointestinal tract) and basophils (found mostly in the blood).
These cells then release a series of chemicals called mediators, which cause the sneezing, runny nose, itching and other symptoms of an allergic reaction. Probably the best known of these mediators is histamine.
Because some mediators work very quickly, symptoms that are caused by the mediators occur almost instantly after exposure to an allergen. These symptoms might include dripping from the nose or eyes, sneezing, wheezing, hives, or itching.
Shortly after your mediators have been released, a type of white blood cell called an called an eosinophil begins to be attracted to the site of the reaction. Eosinophils can accumulate over the course of several days or longer. The result is a more intense reaction with congestion of the tissue - symptoms might include nasal congestion (a stuffy nose), persisting asthma symptoms, or rashes.
Reactions can be most obvious where the allergens are in a high concentration. So, if you inhale an allergen, you’ll experience most of your symptoms within the nose and upper airways. If an allergen enters the bloodstream, its effects can be more far reaching: A food allergen can cause not only gastrointestinal problems, but also hives or other skin rashes.
Who’s At Risk?
If you have allergies, you’re in good company. The National Institute of Allergy and Infectious Diseases estimates that 50 million Americans - one in five of us - has some form of regular allergy.
- Nearly half of people with allergies - at least 10 percent of the entire U.S. population - have hay fever, medically known as allergic rhinitis. This reaction to outdoor airborne allergens like pollens occurs on a seasonal basis. A second type of airborne allergy called perennial allergic rhinitis occurs year-round and is more of a response to persistent indoor allergens such as pet dander or dust mites.
- Roughly 5 percent to 10 percent of the U.S. population - at least 15 million Americans - have asthma. Up to 90 percent of children with asthma also have allergies.
- About half of us get a reaction to poison ivy , with about 50 million affected yearly.
- Approximately 6 million to 10 million Americans are allergic to cats or other pets.
- About 2 million have had allergic reactions to insect stings.
- Approximately one in 12 children under age 6 have symptoms of food allergies, but only about one in 25 has confirmed food allergies. Between 1 percent and 2 percent of adults have food allergies.
All told, allergies rank sixth in cost on the list of chronic diseases in the United States. Each year, Americans lose 31/2 million work days because of allergies at a cost of $639 million. Children lose about 2 million school days. Allergies cost about $2 billion a year in treatment, tests, medications and allergy shots, and require nearly 81/2 million physician visits.
How people become allergic is still something of a mystery. Experts know it begins with what is called sensitization - a period that ranges from a few weeks to several decades, in which repeated exposure to a particular allergen activates the immune system to attempt to fight what is usually an innocuous substance. Because this period of sensitization varies so much, one person can develop a contact allergy during infancy while another may not be sensitized until adulthood. Still, most allergies become apparent in childhood - especially inhalant reactions like hay fever. Some children can outgrow one allergy only to develop another later in life. Adults seldom develop new allergies after age 40, but rarely outgrow those they have, although sometimes reactions become less severe with age.
Regardless of when they begin or what symptoms they cause, most allergies seem to be caused by the same risk factors:
- Heredity. If you have allergies, chances are you’re carrying on a family tradition. A child with one allergic parent has a 30 percent to 50 percent chance of developing allergies, although he or she may get a different type of allergy than his or her folks. A child’s chances of developing allergies rise as high as 60 to 80 percent if both parents have them. Still, children from the same family may not get the same allergies. Studies show that only 25 percent to 50 percent of identical twins share the same type of allergy. In many cases, one twin will have allergies and the other won’t. This means environment is an important factor, too.
- Environment. While your genes make you more vulnerable to an allergy, it’s your environment that sets it all in motion. Scientists are still working out the details of what in the environment contributes to the development of allergies. Of course, your environment exposes you to allergens. But the environment also probably plays a role early in infancy, or even during fetal development. Children who are frequently exposed to certain infectious agents during early infancy have immune systems that emphasize the development of type 1 helper T-cells and IgG antibodies, which do not provoke allergy systems. Bolstering this “set of weapons” shifts resources away from the allergy branch of defense, making it less active and making these children less liley to have allergies. (This same shift in focus by your immune system is the change that occurs when you get allergy shots.) Some scientists believe that exposures to pets, dirt, and the variety of contaminants and infectious agents that may come along with these exposures is the reason that farm children develop allergy less frequently. Some scientists also think this why young children who live with cats or dogs may have fewer allergies overall. Doctors have not developed any specific recommendations to parents that are based on this thinking (for example, doctors do not generally recommend that you purposefully “expose” your child to animals in infancy), but further research is being conducted.
Revision date: June 11, 2011
Last revised: by Sebastian Scheller, MD, ScD