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Allergies: Overview

Last Update: April 23, 2020; Next update: 2023.


Allergies arise if the body's immune system overreacts to foreign substances (allergens) that are usually harmless in most people, such as pollen or certain foods. In some cases the symptoms are quite mild, but they can also be a real nuisance and have a considerable impact in everyday life.

There are various treatment options for allergies. Some things that trigger allergies (allergens) are easy to avoid, whereas others aren't. Common allergens include the following:

  • Pollen
  • House dust mites
  • Pets and farm animals
  • The venom (poison) in insect stings and bites
  • Foods
  • Medication
  • Contact allergens (e.g. metals or fragrance ingredients)
  • Mold

Allergic reactions most often occur on the skin and in the airways and mucous membranes. The symptoms usually start straight away after contact with the allergen, but it can also take a few hours or days for them to appear.

Allergies are sometimes referred to as hypersensitivities. But some hypersensitivities have nothing to do with allergies. These include certain kinds of food hypersensitivity (intolerances). Because the symptoms are often similar, the doctor should carry out a thorough medical examination. If it turns out that the symptoms are being caused by an allergic reaction, allergy treatments can be used.


These are typical symptoms of allergies:

  • Runny nose, coughing and sneezing
  • Breathing problems
  • Watery eyes
  • Itching
  • Rash
  • Swollen mucous membranes
  • Gastrointestinal (stomach and bowel) problems

Allergic reactions usually occur in the areas of the body that come into direct contact with the allergen. For instance, foods can cause itching and swelling in the mouth and on the tongue. The first part of the body that pollen comes into contact with is the upper respiratory tract, resulting in a runny nose and sneezing.

Seasonal hay fever is only noticeable when pollen is in the air. Dust mite allergies and pet allergies can be a problem all year round because the substances that trigger them (the "allergens") aren't only found in the air in the spring or summer months. Although allergies to medications and insect venom are only triggered in certain situations, they are often more severe than other kinds of allergies.

Allergies to foods, insect venom or certain medications can lead to an anaphylactic reaction. This is a medical emergency and can become life-threatening. It happens when the allergic reaction starts affecting other parts of the body too – such as the digestive tract, the airways or the cardiovascular system (heart and blood vessels). Anaphylactic reactions can cause things like itching, swelling, stomach ache, nausea, vomiting, confusion, drowsiness, breathing problems and a sudden drop in blood pressure.

Causes and risk factors

The tendency to have allergies often runs in families. The medical term for this is "atopy." Whether or not someone actually develops an allergy will depend on various things, including environmental factors. Allergies and related conditions such as asthma may be more likely to develop in people who are exposed to air pollution and cigarette smoke, for instance.

In allergic reactions, the body's immune system reacts to certain foreign proteins (allergens) that are usually harmless in most people. It treats them like germs or foreign bodies. For instance, although dust mites and foods generally don't cause any problems in the body, they trigger a reaction in people who are allergic to them. The immune system produces antibodies (IgE antibodies) to fight these substances because it considers them to be harmful.

These antibodies are made when the body first comes into contact with the allergen, and they then attach to certain kinds of cells. This process is called “sensitization.” If the body comes into contact with the allergen again, these cells release chemical substances, including histamines. These substances then lead to allergic reactions such as sneezing, coughing and skin rashes.

Illustration: How allergies develop – as described in the article

How allergies develop


About 1 out of 3 people in Germany are diagnosed with an allergy at some point in their lives. An estimated 20 to 25% of all children and teenagers have hay fever, asthma or eczema. The number of people with allergies has increased in the last thirty years.


Allergies usually start in childhood or puberty, and often don't go away again. Some allergies get better in adulthood, though. For instance, certain food allergies (e.g. allergies to milk or chicken eggs) may disappear again – but peanut allergies generally don't go away. So whether or not an allergy can get better depends on what triggers it.

Allergy symptoms are often mild at first and then become more severe over time. They sometimes lead to other medical conditions. If people have hay fever for many years, the allergy might “move down” into the lower airways (lungs) too, causing asthma. Allergies can also increase the likelihood of developing eczema.

People who are allergic to pollen, for instance, might also be allergic to certain foods. This is known as "cross-reactivity." It occurs because the different allergens have similar proteins in them, and the immune system reacts to both of them.


If you see a doctor, you will first be asked about your symptoms, your everyday life circumstances, and medical history. Various tests can be used to find out whether you have an allergy:

  • Skin prick test: This test is used if it's thought that someone might have an allergy that leads to a reaction immediately after the person comes into contact with the trigger, such as hay fever. The test involves placing small drops of various allergens on the skin of your forearm, leaving enough space between them, and then pricking the surface of your skin where the allergens are, so the substances go into the skin. If your skin becomes red and swollen, like a big mosquito bite, you're having an allergic reaction.
Illustration: Skin prick test - as described in the information

Skin prick test

  • Patch test: This test is used if you are thought to have an allergy where the symptoms only start one half to three days after contact with the allergen. It involves putting a patch containing the suspected allergen on your back for one to two days, and checking to see whether the skin becomes swollen, red and itchy.
  • Provocation test: In provocation tests, the body is exposed to various allergens in order to see whether there’s a reaction. If, for instance, it is thought that you might have allergic rhinitis (an allergy-related inflammation of the inside of the nose), extracts of potential allergens are placed on the lining of your nose using a nasal spray or drops. If the lining of your nose becomes swollen, you sneeze and your nose starts running, you are likely to have allergic rhinitis. A provocation test is usually only done if the other tests haven't led to any clear results.

Sometimes it may be necessary to do a blood test that is sent to a lab to find antibodies to certain allergens: for example, if testing the skin is too risky or if the outcome was not clear. Not all allergens can be tested using skin tests, either.

Allergy tests are done by doctors who have specialized in allergies. These include dermatologists, ENT (ear, nose and throat) doctors, pulmonologists (lung specialists) and pediatricians.


It's often not possible to prevent an allergy from developing. Certain measures may lower the risk a little. For instance, allergic asthma is somewhat less common in children who are exclusively breastfed in the first four to six months of their lives. But research has found that this makes little to no difference to other symptoms, such as allergic rhinitis. Non-smokers and children who grow up in smoke-free environments are also less likely to develop allergies.

If you have a high risk of developing an allergy, it might be a good idea to not get a pet cat in order to avoid developing a cat allergy. But if you don't have a greater risk, then there's no reason not to have a pet.


The following approaches can help to relieve allergy symptoms:

  • Avoid contact with allergens: How well this works will mainly depend on the type of trigger. It’s usually quite easy to avoid contact with things like certain foods or chemical substances. It’s almost impossible to avoid pollen, though. But pollen calendars and forecasts, for instance on the internet, can give you an idea of the pollen count in certain regions. People with dust mite allergies can somewhat lower their exposure to dust mites in their home by taking precautions such as cleaning regularly, using mite-proof bedding and removing "dust traps."
  • Medication: Antihistamines or steroids are often used, especially for allergic rhinitis and allergic conjunctivitis (an allergy-related eye inflammation). These medications are available in various forms, including tablets, nasal sprays and injections. Steroid lotions or creams can be used to treat allergic skin reactions.
  • Allergen-specific immunotherapy (desensitization): A bit like with vaccines, this treatment approach involves exposing people to small amounts of the allergen. Here it is done at regular intervals by either injecting the allergen under your skin or placing it under your tongue in the form of a tablet or drops. It takes about three to five years to complete allergen-specific immunotherapy. This approach is mainly used for the treatment of allergies to pollen (hay fever), dust mites and insect venom.

There is currently no evidence that herbal or homeopathic products can help in the treatment of allergies. This is also true for acupuncture.

Further information

Many researchers are trying to find out more about how the immune system and environmental factors interact with each other, with the aim of discovering new ways to prevent and treat allergies. The German Research Center for Environmental Health in Munich (the Helmholtz Center) provides up-to-date information on allergy research.

When people are ill or need medical advice, they usually go to see their family doctor first. Read about how to find the right doctor, how to prepare for the appointment and what to remember.


© IQWiG (Institute for Quality and Efficiency in Health Care)
Bookshelf ID: NBK447112


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