Allergen-specific immunotherapy (desensitization) in the treatment of allergies

Photo of a doctor and patient (PantherMedia / Iakov Filimonov)

People who have hay fever, a dust mite allergy or an animal allergy sneeze a lot, and have a runny or stuffy nose. Many of those who have very severe and bothersome symptoms try out allergen-specific immunotherapy (desensitization). This treatment aims to make the immune system “get used to” the substances triggering the allergy, so that it no longer reacts as strongly to them.

The goal of allergen-specific immunotherapy is to reduce allergy symptoms in the medium to long term. It has to be repeated regularly and takes quite a long time to start working. This treatment is also known as desensitization. It aims to do exactly that: make the immune system less sensitive. In people who are allergic to something, their body is oversensitive or hypersensitive to an allergen (the substance that causes their allergic reaction). Their body produces antibodies to fight the allergen, even though the allergen is harmless. These antibodies are part of a chain reaction that leads to allergy symptoms.

In allergen-specific immunotherapy, people are given allergen extracts to try to train their body 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.

Which allergies can be treated using allergen-specific immunotherapy?

Allergen-specific immunotherapy can't be used for every allergy. It is called "specific" because the allergen extract has to be tailored to the individual person's allergic response. These allergen extracts aren’t available for every allergy trigger. There are currently allergen extracts that have been approved for the treatment of allergies caused by

  • 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 are allergic to cats or mold. But unlike with the approved allergen extracts, there is no guarantee that unapproved ones will be effective enough.

What does the treatment involve?

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 increased gradually until their personal maximum dose is reached, which is then given at regular intervals. It should be as high as possible without causing too much of an allergic reaction. So both the person getting the treatment and the doctor need to watch very carefully for reactions.

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 used less often, usually once a month. The drops or tablets for sublingual immunotherapy are typically taken once a day.

Can allergen-specific immunotherapy help?

Both kinds of allergen-specific immunotherapy (SCIT and SLIT) are effective treatments. Many studies have shown that they can relieve the symptoms of allergic rhinitis and reduce the need for medication. Sometimes there is already a noticeable improvement in symptoms within the first year. But it typically takes at least two to three years before the treatment reaches its full effect.

There's no guarantee that this treatment will work, though. In some people it doesn’t provide any relief from sneezing, swollen mucous membranes, and itchy and watery eyes. Sometimes the symptoms 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.

Research has shown that immunotherapy is effective in the treatment of hay fever and year-round (perennial) allergic rhinitis. 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 animals. Some studies suggest that it could also be effective in people who are allergic to cats, though.

What are the side effects?

Mild and temporary side effects are very common in allergen-specific immunotherapy. This is to be expected when a person is given a substance that they are usually allergic to. The likelihood of side effects depends on things like

  • how sensitive they are to the allergen,
  • how high the dose is, and
  • which drug is used.

About 10 to 60 out of 100 people develop a mild skin rash at the site of injection. When allergens are placed under the tongue, itching and swelling in the mouth are common. Up to 50 out of 100 people have allergic reactions such as sneezing, watery eyes or asthma symptoms at least once. Tiredness and headaches can occur too. 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 also 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.

Rare: Anaphylactic reaction

The biggest concern is that allergen-specific immunotherapy will cause an extreme allergic response called an anaphylactic reaction (sometimes referred to as "anaphylactic shock"). Among other things, this can lead to severe itching, nausea, breathing difficulties, circulation problems, and may even result in anaphylactic shock. In severe cases an anaphylactic reaction is life-threatening, More severe reactions 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. There are only rough estimates of the risk. In studies, fewer than 1 out of 1,000 people who were given 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 have had an allergic reaction to an injection in the past, it's important to tell your doctor about it before starting allergen-specific immunotherapy.

Which is more effective: Injections, drops or tablets?

It is not yet possible to say for sure whether one type of allergen-specific immunotherapy is better than the others. Several studies have compared the treatments with each other, though. They suggest 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.