Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance.
Normally, the immune system —which includes antibodies, white blood cells, mast cells, complement proteins, and other substances—defends the body against foreign substances (called antigens ). However, in susceptible people, the immune system can overreact when exposed to certain substances (allergens) in the environment, foods, or medications, which are harmless in most people. The result is an allergic reaction. (Allergens are molecules that the immune system can identify and that can stimulate a response by the immune system.) Some people are allergic to only one substance. Others are allergic to many.
Allergens may cause an allergic reaction when they land on the skin or in the eye or are inhaled, eaten, or injected. An allergic reaction can occur in several ways:
In many allergic reactions, the immune system , when first exposed to an allergen, produces a type of antibody called immunoglobulin E (IgE) . IgE binds to a type of white blood cell called basophils in the bloodstream and to a similar type of cell called mast cells in the tissues. The first exposure may make people sensitive to the allergen (called sensitization) but does not cause symptoms. When sensitized people subsequently encounter the allergen, the basophils and mast cells with IgE on their surface release substances (such as histamine, prostaglandins, and leukotrienes) that cause swelling or inflammation in the surrounding tissues. Such substances begin a cascade of reactions that continue to irritate and harm tissues. These reactions range from mild to severe.
Some people have an inherited tendency to produce a lot of IgE (a condition called atopy) and may overreact to some antigens that cause hay fever, asthma, skin problems, or a food allergy.
Latex is a fluid that comes from the rubber tree. It is used to make rubber products, including some rubber gloves, condoms, and medical equipment such as catheters, breathing tubes, enema tips, and dental dams.
Latex can cause the immune system to produce antibodies to IgE, which can lead to allergic reactions, including hives, rashes, and even severe and potentially life-threatening allergic reactions called anaphylactic reactions. However, the dry, irritated skin that many people develop after wearing latex gloves is usually the result of irritation and not an allergic reaction to latex.
People may be at risk of becoming sensitive to latex if they
For unknown reasons, people who are allergic to latex are often allergic to bananas and sometimes other foods such as kiwi, papaya, avocados, chestnuts, potatoes, tomatoes, and apricots.
Doctors may suspect latex allergy based on symptoms and the person's description of when symptoms occur, especially if the person is a health care professional. Blood or skin tests are sometimes done to confirm the diagnosis.
People who are allergic to latex should avoid it. For example, health care professionals can use gloves and other products that are latex-free. Most health care institutions provide them.
Genetic and environmental factors work together to contribute to the development of allergies.
Genes are thought to be involved because specific mutations are common among people with allergies and because allergies tend to run in families.
Environmental factors also increase the risk of developing allergies. These factors include the following:
On the other hand, exposure to various antigens, such as bacteria and viruses and foods (including peanuts), during childhood may strengthen the immune system. Such exposure may help the immune system learn how to respond to allergens in a way that is not harmful and thus help prevent allergies from developing. An environment that limits a child's exposure to bacteria and viruses—commonly thought of as a good thing—may make allergies more likely to develop. Exposure to microorganisms is limited in families with fewer children and cleaner indoor environments and by the early use of antibiotics.
Microorganisms live in the digestive tract, in the respiratory tract, and on the skin, but which microorganisms are present varies from person to person. Which ones are present appears to affect whether and which allergies develop.
Allergens that most commonly trigger allergic reactions include
House dust mites live in the dust that builds up in carpets, bedding, soft furnishings, and soft toys.
Most allergic reactions are mild, consisting of watery and itchy eyes, a runny nose, itchy skin, and some sneezing. Rashes (including hives) are common and often itch.
Hives , also called urticaria, are small, red, slightly elevated areas of swelling (wheals) that often have a pale center. Swelling may occur in larger areas under the skin (called angioedema ). Swelling is caused by fluids leaking from blood vessels. Depending on which areas of the body are affected, angioedema may be serious, particularly when it occurs in the throat or airways.
Allergies may trigger attacks of asthma .
Certain allergic reactions, called anaphylactic reactions , can be life threatening. The airways can narrow (constrict), causing wheezing, and the lining of the throat and airways may swell, interfering with breathing. Blood vessels can widen (dilate), causing a dangerous fall in blood pressure.
Photo provided by Thomas Habif, MD.
Doctors first determine whether a reaction is allergic. They may ask
To help determine whether a reaction is allergic, doctors sometimes do blood tests to detect a type of white blood cell called eosinophils . Eosinophils, although present in everyone, are usually produced in greater numbers when an allergic reaction occurs. However, the usefulness of this test is limited because other eosinophilic disorders can cause the number of eosinophils to increase, and a normal number does not exclude the presence of an allergy.
If it seems likely that a person's symptoms are caused by an allergy, the main goal is to identify the specific allergen. Often, the person and doctor can identify the allergen, or at least the type of allergen, based on when the allergy started and when and how often the reaction occurs (for example, during certain seasons or after eating certain foods).
Skin tests and a blood test called the allergen-specific serum IgE test can also help doctors detect the specific allergen. However, these tests may not detect all allergies, and they sometimes indicate that people are allergic to an allergen when they are not (called a false-positive result).
Skin tests are the most useful way to identify specific allergens. An allergen applied to or injected into the skin should cause a skin reaction in people who are allergic to it. There are 2 types of skin tests:
To help ensure that the results of these skin test are reliable, doctors give 2 control solutions in addition to the test solution (which contains the suspected allergen). The control substances are
Usually, doctors give several test solutions. These are dilute solutions, each with one specific antigen. Commonly used antigens include pollens (of trees, grasses, or weeds), molds, dust mites, animal dander, insect venom, foods, and some antibiotics. Doctors choose the antigens for this test based on which substances they suspect to be the cause.
Usually, a skin prick test is done first. A drop of each of the control and test solutions is placed on the person’s skin, which is then pricked with a needle through the drop. The skin prick test can identify most allergens.
If no allergen is identified, an intradermal test may be done. For this test, a tiny amount of each of the control and test solutions is injected into the person’s skin through a needle. This type of skin test is more sensitive and more likely to detect a reaction to an allergen.
If the person is allergic to one or more of the allergens in the test solution, the person has a wheal and flare reaction, indicated by the following:
Before skin tests are done, people are asked to stop taking medications that may suppress a reaction in a person who actually has an allergy to allergens in the test solution. These medications include
Some doctors also do not test people who are taking beta-blockers because if such people have an allergic reaction in response to the test, the consequences are more likely to be serious. In addition, beta-blockers may interfere with the medications used to treat serious allergic reactions.
The allergen-specific serum IgE test, a blood test, is used when skin tests cannot be used—for example, when a rash is widespread. This test determines whether IgE in the person's blood binds to the specific allergen used for the test. If binding occurs, the person has an allergy to that allergen.
For provocative testing people are directly exposed to a small amount of the suspected allergen. This test is usually done when people must document their allergic reaction—for example, for a disability claim. It is sometimes used to diagnose a food allergy . If doctors suspect an exercise-induced allergy , they may ask the person to exercise. If doctors suspect an allergy triggered by cold , they may place an ice cube on the person's skin to see if a rash develops.
Avoiding or removing an allergen, if possible, is the best approach. Avoiding an allergen may involve the following:
People with allergies should avoid or minimize exposure to certain other irritants that can make allergic symptoms worse or cause breathing problems. These irritants include the following:
Allergen immunotherapy, usually allergy shots (injections), can be given to desensitize people to the allergen when some allergens, especially airborne allergens, cannot be avoided and the drugs used to treat allergic reactions are ineffective.
With allergen immunotherapy, allergic reactions can be prevented or reduced in number and/or severity. However, allergen immunotherapy is not always effective. Some people and some allergies tend to respond better than others.
Immunotherapy is used most often for allergies to
When people are allergic to unavoidable allergens, such as insect venom, immunotherapy helps prevent anaphylactic reactions . Sometimes it is used for allergies to animal dander, but such treatment is unlikely to be useful. Immunotherapy for peanut allergy is available, and immunotherapy for other food allergies is being studied.
Immunotherapy is not used when the allergen, such as penicillin and other medications, can be avoided. However, if people need to take a medication that they are allergic to, immunotherapy, closely monitored by a doctor, can be done to desensitize them.
In immunotherapy, tiny amounts of the allergen are either injected under the skin or given by mouth, depending on the specific allergen. The first dose is so small that even an allergic person does not react to it. However, the small dose starts to get the person's immune system used to the allergen. Then the dose is gradually increased. Each increase is so small that the immune system still does not react. The dose is increased until the person is not reacting to the same amount of allergen that once caused symptoms. This dose is the person's maintenance dose. A gradual increase is necessary because exposure to too much allergen too soon can cause an allergic reaction. Injections are usually given once or twice a week until the maintenance dose is reached. Then injections are usually given every 2 to 4 weeks. The procedure is most effective when maintenance injections are continued throughout the year, even for seasonal allergies.
Alternatively, doses of the allergen may be placed under the tongue (sublingual) and held there for a few minutes, then swallowed. The dose may be gradually increased, as for injections. The sublingual dose may be given every day or up to 3 times a week. Extracts for grass pollen, ragweed, or house dust mite, placed under the tongue, can be used to help prevent hay fever ( allergic rhinitis ).
Immunotherapy for peanut allergy may also be given by mouth (see Food Allergy, Prevention ). The person receives the first several doses of the allergen over the course of a single day while in a doctor's office or clinic. The person then takes the allergen at home. Each time the dose is increased, the first dose of the higher dosage is given under a doctor's supervision.
Allergen immunotherapy may take 3 years to complete. People who develop allergies again may need another longer course (sometimes 5 years or more) of immunotherapy.
Avoiding the allergen is the best way to treat as well as prevent allergies.
If mild symptoms occur, antihistamines are often all that is needed. If they are ineffective, other medications, such as mast cell stabilizers and corticosteroids, may help. Nonsteroidal anti-inflammatory drugs (NSAIDs) are not useful, except in eye drops used to treat allergic conjunctivitis .
Severe symptoms, such as those involving the airways (including anaphylactic reactions
The medications most commonly used to relieve the symptoms of allergies are antihistamines. Antihistamines block the effects of histamine (which triggers symptoms). They do not stop the body from producing histamine.
Antihistamines are available as
Which antihistamine is used depends on the type of allergic reaction. Some antihistamines are available without a prescription (over-the-counter, or OTC), and some require a prescription.
Side effects of antihistamines include anticholinergic effects , such as drowsiness, dry mouth, blurred vision, constipation, difficulty with urination, confusion, and light-headedness (particularly after a person stands up), as well as drowsiness.
Some antihistamines are more likely to cause drowsiness (sedation) than others. People should not take these antihistamines if they are going to drive, operate heavy equipment, or do other activities that require alertness. Antihistamines that cause drowsiness should not be given to children under 2 years old because they may have serious or life-threatening side effects. These antihistamines are also a particular problem for older adults and for people with glaucoma, benign prostatic hyperplasia, constipation, or dementia because of the medications’ anticholinergic effects. People with cardiovascular disease should consult their doctor before taking antihistamines, even OTC ones.