Immunotherapy for allergic asthma

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,
• animals, 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 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 injected into the skin.
• 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 injected 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.

Research has shown that people who have asthma can benefit from allergen-specific immunotherapy. Both injecting the extract solution and possibly placing it under the tongue can help relieve the symptoms and prevent asthma attacks.

According to a recent review of over 50 studies, it’s not possible to reliably assess the effectiveness of sublingual immunotherapy.

Research on subcutaneous immunotherapy found that:

• Without immunotherapy: Asthma symptoms got worse after a few years in about 60 out of 100 people who did not have immunotherapy.
• With immunotherapy: Symptoms got worse in about 30 out of 100 people who had immunotherapy.

In other words, allergen-specific immunotherapy prevented asthma from getting worse in 30 out of 100 people.

The treatment also had another effect: About 20 out of 100 people who had allergen-specific immunotherapy needed less asthma medication overall.

The effect of allergen-specific immunotherapy can last for several years. It then wears off in some people.

Immunotherapy is also effective against allergic rhinitis ( allergy symptoms that affect the upper airways, including itchy or watery eyes, sneezing, and a stuffy or runny nose).

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. Placing allergens under the tongue often causes itching and swelling in the mouth. 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 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.

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 circulatory shock (not enough blood flowing through the body). In the worst case, anaphylactic reactions can be 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, precautionary measures are taken: People who are given an allergen extract have to stay at the doctor's office for at least 30 minutes afterwards. Your doctor should describe the symptoms of an anaphylactic reaction to you, and make sure your family is informed too. If you then start showing signs of this reaction, 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.

It's not yet possible to say for sure whether one type of allergen-specific immunotherapy is better than the other. Several studies have compared the two treatment approaches with each other. 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.