If you have hay fever, or are allergic to dust mites or animals, immunotherapy can be used to weaken your body's response to the substance causing the allergy, known as the allergen. This treatment allows the immune system to "get used to" (become desensitized to) the allergen. That can relieve symptoms.

The goal of allergen-specific immunotherapy is to reduce allergy symptoms in the medium to long term. It takes about three years. Immunotherapy can be used in both children and adults.

This treatment is also known as desensitization. It aims to do exactly that: make the immune system less sensitive to the allergen by allowing it to "get used to" it. An allergy is an exaggerated reaction to a substance that is actually harmless. The body produces antibodies to protect itself – against what it wrongly believes to be harmful allergens. These antibodies are part of a chain reaction that leads to allergy symptoms.

In immunotherapy, the body is exposed to extracts of the allergen with the aim of training it to react differently: It’s a bit like being “vaccinated” against your own allergy. The allergen extracts can be injected or taken as tablets or drops.

Allergen-specific immunotherapy can't be used for every allergy. Allergen extracts that have been approved for use in immunotherapy are available for

• grass pollen, grain pollen and weed pollen,
• tree pollen,
• dust mites, and
• wasp and bee venom.

It's also possible to do immunotherapy with allergen extracts that haven't been approved – for example, if you're allergic to cats or mold. But unlike with the approved allergen extracts, there's no guarantee that unapproved ones will be effective enough.

In specific immunotherapy, the person is repeatedly given a very small amount of the allergen. To allow their body to get used to the allergen, only a very small amount of the substance is used at first. This dose is gradually increased until their personal maximum dose is reached, which is then given at regular intervals.

There are two types of immunotherapy:

• Subcutaneous immunotherapy (SCIT): The allergens are delivered by injection (sometimes referred to as "allergy shots").
• Sublingual immunotherapy (SLIT): The allergens are given in the form of drops or tablets, and are kept under your tongue (sublingual) for one or two minutes before they are swallowed.

People who have subcutaneous immunotherapy (injections) have to go to the doctor regularly. The allergen is given every week or two at first. Later on it's given less often, usually once a month. The drops or tablets for sublingual immunotherapy are typically taken once a day at home by yourself.

Both kinds of allergen-specific immunotherapy (SCIT and SLIT) are effective treatments. Many studies have shown that they can relieve the allergy symptoms and reduce the need for medication. These treatments can already lead to an improvement in sneezing, a stuffy nose, and itchy and watery eyes in the first year. But it typically takes at least two to three years before the treatment reaches its full effect.

Immunotherapy works for hay fever and for year-round allergic rhinitis (allergy symptoms in the upper airways, including sneezing and a stuffy or runny nose). In other words, it is suitable for people who are allergic to tree, weed and grass pollen, as well as for people who are allergic to dust mites. There isn’t as much good research on how well it works in people who are allergic to mold or animals. But some studies suggest that it could also be effective in people who are allergic to cats.

There's no guarantee that this treatment will work, though. It doesn't relieve the symptoms in some people. And the symptoms sometimes become worse again after the end of treatment.

But allergen-specific immunotherapy has many important advantages over all other allergy treatments: It can prevent the original allergy from developing into allergies to other substances (cross-reactivity), or from developing into asthma. And it's the only treatment approach that directly targets the cause of the symptoms. So if allergen-specific immunotherapy is successful, less or no medication is needed.

Which is most effective: injections, drops or tablets?

Research suggests that injections could relieve the symptoms somewhat more effectively than medication placed under the tongue in the form of tablets or drops. But because the treatment approaches are at least similarly effective, the treatment decision can be based on personal preferences.

About half of the people who have this treatment experience mild side effects. This is to be expected when a person is given a substance that they are usually allergic to. The side effects include the following:

• Mild rash at the site of the injection
• Allergic reactions such as sneezing, watery eyes or mild asthma symptoms
• Itching and swelling in the mouth
• Tiredness and headaches

These side effects are usually mild and temporary. They only rarely need to be treated.

If severe side effects occur, the dose is reduced again. It is then a good idea to rule out other possible causes too. Should other causes of these symptoms be found and treated, the dose might be increased again.

Immunotherapy can also be started during the pollen season. Some people worry that the treatment might be harder on their body at that time of year, and then cause more allergy symptoms. But studies show that the side effects are about the same as when the immunotherapy is started in winter.

Very rare: Anaphylactic reaction

In very rare cases, immunotherapy can cause an extreme allergic reaction known as an anaphylactic reaction. This can lead to problems like severe itching, nausea, breathing difficulties, circulation problems, and may even result in anaphylactic shock. These symptoms require immediate medical attention. An adrenaline (epinephrine) injection is usually given in order to quickly limit the dangerous overreaction occurring in the body.

Anaphylactic reactions are very rare in people who are otherwise healthy. In studies, fewer than 1 out of 1,000 people who were given immunotherapy injections had a reaction that needed to be treated with adrenaline. Because of this small risk, though, precautionary measures are taken: Every time you're given an allergen extract, you must stay in the doctor’s office for at least 30 minutes afterwards. Your doctor should also describe the symptoms of an anaphylactic reaction to you, and make sure your family is informed too. If you then start showing signs later on, you can seek medical help immediately.

The risk of an anaphylactic reaction is much higher in some people: For instance, certain heart problems can increase the risk, and if you're taking beta blockers it's harder to treat possible side effects. So people with cardiovascular (heart and blood vessel) disease can usually only have allergen-specific immunotherapy if beta blockers can be replaced with other medications. People with severe asthma can have immunotherapy if their asthma is well controlled and stable. If you've had an allergic reaction to an injection in the past, it's important to tell your doctor about it before starting allergen-specific immunotherapy.