Allergic asthma is the most prevalent form of asthma in the general population, however, there is no cure on the market. Current standard therapies for all asthma subtypes involve treating the symptoms as they come and not the underlying causes. Treatment of the allergies underlying exacerbations, or asthma attacks, gives hope for a better quality of life. Allergen immunotherapy is a promising treatment for asthma, with many clinical trials completed and ongoing. Below is a discussion of allergen immunotherapy as it applies to asthma. 

Introduction

Asthma is a common lung condition that results in bouts of breathing difficulties (exacerbations), caused by inflammation of the lungs. During these exacerbations, the airways become inflamed and narrow, allowing less air to pass through. This, paired with the increased mucus production, makes asthma not only uncomfortable but potentially life-threatening. There is no cure for asthma, however, there are ways to manage the disease and ease symptoms.¹ 

Asthma can have a plethora of different causes:²

• Genetics
• Tobacco smoke
• Pollution
• Allergies
• Exercise 
• Infections

The most common presentation of asthma is allergic asthma, where exposure to an allergen causes an inflammatory reaction that results in an asthma attack.³ This happens when an allergen (such as dust mites, pollen, or animal hair) gets in the airways and activates immune cells. The result is a series of cell signalling mechanisms that cause the airway to swell and increase mucus secretion in the lung. This limits the amount of air that can enter and exit the lungs, leading to breathing difficulties. One way to treat this asthma is to treat the allergy that is responsible for triggering asthma exacerbations. 

Allergen immunotherapy (AIT) is a medical treatment for allergies, which involves exposing the body to slowly increasing concentrations of an allergen, to desensitise the immune system and hopefully get rid of the allergic response altogether. Therefore, it is a promising way to decrease exacerbations in allergic asthma. 

Mechanism of action 

AIT involves exposing the patient to small doses of an allergen and gradually increasing the doses to build tolerance and decrease the severity of the allergy. In terms of asthma, it aims to decrease the number of exacerbations experienced as a result of allergen exposure. In AIT, there are two different ways of exposing the body to the allergen that it is sensitive to: subcutaneous (SCIT) or sublingual (SLIT). 

Subcutaneous immunotherapy

SCIT involves injection of the allergen under the skin. Initially, there is a period where the concentration of allergen injected is increased with each treatment. This is followed by a maintenance period, in which there is a stable concentration of allergen injected periodically in the long term.⁴ After this period, the treatment can be discontinued and its benefits enjoyed. 

Sublingual immunotherapy 

SLIT involves the administration of the allergen under the tongue. The therapy must stay there to absorb for a few minutes and can then be swallowed or spat out.⁴ The administration routine is the same as for SCIT, with both a build-up and maintenance phase, after which the benefits of the therapy persist. 

How does this impact the lungs?

When the body is exposed to the allergen, it mistakenly perceives it as an immunological threat, thus promoting an inflammatory response. In allergic asthma, exposure to an allergen results in the activation of specific allergy-related immune cells (T Helper 2 Cells), which cause its symptoms. AIT aims to shift the reaction to the allergen to other immune cells not associated with the allergic response (for example the T helper 1 Cells) over time. Following therapy, when there is exposure to an allergen, instead of triggering an inflammatory response via T Helper 2 cells that causes the airways to swell, there should be a less severe reaction or no reaction at all. 

Effectiveness of AIT in reducing asthma exacerbations

There are a few markers of what makes a successful therapy for asthma. Firstly, the use of asthma medications during or after a therapeutic intervention is an indicator of how well it has worked. In this case, it can be indicative of the severity of attacks or their frequency. Secondly, the quality of life can be assessed to decide the wider physical and psychosocial effects of the therapy.  

Research has shown that AIT can reduce medication use and increase the quality of life in those with asthma caused by allergies to house dust mites and pollen.^4,5,6 There is also evidence that AIT can decrease the number and severity of exacerbations.^7,8 However, the results of clinical trials for this treatment are still highly variable.⁵ 

Factors influencing effectiveness

AIT is a treatment designed to help with the allergic reactions underlying asthma exacerbations, hence, the best candidates for this therapy are those with allergic asthma. AIT also has the highest efficacy in cases where the asthma is not severe, as there is less risk associated with allergen exposure.⁵ 

Below are some further factors which could influence the results of this therapy:⁹

• Administration during pollen season can affect dosages of those with pollen allergy
• Antigen serum composition 
• Comorbidities with other conditions (such as rhinitis)
• Inconsistent dosage or dosage escalations

Safety and adverse reactions

AIT is a relatively well-tolerated form of treating allergies. However, it carries risks that are general or specific to the mode of administration.⁹ 

Subcutaneous and sublingual immunotherapy

• Local allergic reaction
• Systemic allergic reaction
• Anaphylaxis

Sublingual immunotherapy only

• Diarrhoea 
• Nausea
• Abdominal pain

Local allergic reactions are a common side effect of this treatment and are usually not severe but may be uncomfortable. More serious systemic reactions can occur with incorrect dosage, or incorrect administration, or can be dependent on the history of the individual.⁸ 

Summary

Asthma is a common but potentially life-threatening condition, which causes the constriction of the airways and periods of breathing difficulties, known as exacerbations. The most common cause of asthma is allergic asthma, in which an allergen (such as dust mites, pollen or animal hair) triggers the immune system and causes exacerbations. There are no current cures for this disease, however, there are ways to manage the symptom frequency and severity. 

Allergen immunotherapy provides a promising disease-altering treatment for allergic asthma. It involves gradually increasing the exposure to an allergen subcutaneously or sublingually, promoting tolerance or desensitisation to it. It has been shown that allergen immunotherapy can improve the symptoms of allergic asthma, decreasing the need for medication and improving overall quality of life. When it comes to asthma exacerbations, it has been shown that it can decrease the frequency and severity of asthma attacks in some but not all cases. This is highly dependent on the individual, with factors such as type of allergy and asthma severity playing a major part in treatment effectiveness. Allergen immunotherapy is generally well tolerated, with non-severe local reactions being common. However, there is the threat of systemic reactions, which are mitigated with close monitoring during treatment. Overall, allergen immunotherapy has potential in the treatment of allergic asthma in some patients, possibly being disease-altering.  

FAQs

Do allergies get worse during immunotherapy?

During the initial build-up stage of the therapy, it can cause local flare-ups at the site of administration and make reactions to allergens worse if you are exposed to them (for example when taking this therapy for pollen allergy-induced asthma during pollen season). 

How quickly does allergy immunotherapy work?

The timeline of the therapy varies between individuals. This said, this is not a “quick fix” therapy and requires consistent administration. The effects of allergen immunotherapy can usually be seen from 6 months onwards. 

What is the best age for allergy immunotherapy?

Allergen immunotherapy has been tested in children as young as 5.¹⁰ It is typically recommended to commence therapy as soon as the allergy diagnosis for best clinical outcomes. 

What is the success rate of allergy immunotherapy?

The success of the therapy relies a lot on various factors, such as consistency in taking doses, genes, and severity of the allergy. Allergen immunotherapy in asthma is also associated with decreasing but not eliminating symptoms, hence it is difficult to judge its success rate, as each study has its definition of success.