Immunotherapy has emerged as a transformative approach in the management of allergic diseases and asthma, offering the potential for long-term disease modification rather than mere symptom control. Unlike conventional pharmacotherapy—which relies on antihistamines, nasal corticosteroids, and bronchodilators to manage acute symptoms—immunotherapy targets the underlying immune dysregulation. By systematically exposing the immune system to increasing doses of specific allergens, this therapy induces tolerance and can lead to sustained clinical improvement even after treatment discontinuation. For patients with moderate-to-severe allergic rhinitis, asthma triggered by aeroallergens, or venom hypersensitivity, immunotherapy represents a cornerstone of modern allergy care.

Understanding Immunotherapy and Its Mechanism

What Is Immunotherapy?

Allergen immunotherapy (AIT), commonly referred to as allergy shots or allergy desensitization, is a therapeutic intervention that modifies the body’s immune response to allergens. It is indicated for IgE-mediated allergic conditions, including allergic rhinitis, allergic asthma, and stinging insect hypersensitivity. The treatment involves administering gradually increasing doses of allergen extracts—either via subcutaneous injection (SCIT) or sublingual tablets/drops (SLIT)—over months to years. The goal is to shift the immune response from a pro-allergic Th2 phenotype toward a tolerogenic profile, characterized by increased IgG4-blocking antibodies, regulatory T cells, and a reduction in mast cell and basophil reactivity.

The Science Behind Desensitization

The immunologic changes induced by AIT are complex and multi-layered. Early during treatment, there is a decrease in mast cell and basophil activation, resulting in reduced release of histamine and other mediators. Over the long term, AIT promotes the generation of allergen-specific regulatory T cells (Tregs) that secrete IL-10 and TGF-β. These cytokines suppress Th2 responses and promote a shift toward IgG4 antibody production. IgG4 competes with IgE for allergen binding, effectively acting as a blocking antibody that prevents cross-linking of IgE on effector cells. Additionally, AIT can decrease tissue infiltration by eosinophils and reduce the late-phase allergic response. These changes are sustained for years following successful treatment, explaining the durable clinical benefit.

Types of Immunotherapy for Allergies and Asthma

Subcutaneous Immunotherapy (SCIT) – Allergy Shots

SCIT is the most established form of AIT and has been used for more than a century. It involves receiving injections of allergen extracts, typically in a healthcare setting, with a build-up phase (weekly or twice-weekly doses) followed by a maintenance phase (monthly doses). SCIT is highly effective for pollen, dust mite, pet dander, and mold allergies. Meta-analyses show significant reductions in symptom scores and medication use in patients with allergic rhinitis and asthma. For asthma specifically, SCIT has been demonstrated to reduce airway hyperresponsiveness and the need for inhaled corticosteroids. A 2022 Cochrane review concluded that SCIT is associated with a moderate-to-large reduction in asthma symptoms and a lower risk of exacerbations (source: Cochrane Review).

Sublingual Immunotherapy (SLIT) – Tablets and Drops

SLIT has gained popularity as a more convenient and safer alternative to SCIT, particularly for grass and ragweed pollen allergies, as well as dust mite allergic rhinitis. SLIT is administered daily at home in the form of rapidly dissolving tablets or liquid drops held under the tongue for 1–2 minutes before swallowing. The sublingual route leverages the tolerogenic nature of oral mucosal immune cells, leading to systemic tolerance. SLIT has a favorable safety profile with a very low risk of systemic anaphylaxis, making it suitable for wider use. For asthma, SLIT has been shown to improve asthma control and reduce exacerbations in patients with concomitant allergic rhinitis. The FDA has approved multiple SLIT tablets for use in the United States, including those for grass, ragweed, and dust mite allergies (source: FDA Allergenics).

The Role of Immunotherapy in Asthma Management

Reducing Asthma Exacerbations

Asthma exacerbations, often triggered by viral infections or allergen exposure, are a major cause of morbidity and healthcare utilization. AIT targets the allergic trigger component, thereby reducing the frequency and severity of exacerbations. A landmark study published in the New England Journal of Medicine demonstrated that dust mite SLIT reduced the rate of moderate-to-severe asthma exacerbations by 42% compared to placebo in patients with allergic asthma not well-controlled by inhaled corticosteroids (source: NEJM). Similar benefits have been observed with SCIT, where meta-analyses report a 40–50% reduction in the risk of exacerbation.

Long-term Asthma Control and Remission

Beyond reducing exacerbations, AIT has the potential to alter the natural course of allergic asthma. Long-term follow-up studies indicate that patients who complete a 3–5 year course of AIT maintain improved asthma control for many years after treatment cessation. Some children with allergic asthma may achieve clinical remission—defined as the absence of symptoms and medication use—after completing AIT. The European Academy of Allergy and Clinical Immunology (EAACI) guidelines recommend consideration of AIT for patients with allergic asthma who have symptoms despite optimal pharmacotherapy and allergen avoidance.

Patient Selection and Eligibility

Who Is a Good Candidate?

Ideal candidates for AIT include patients with documented IgE-mediated allergies to clinically relevant aeroallergens (pollen, dust mites, animal dander, molds) that contribute to their allergic rhinitis or asthma. Candidates should have moderate-to-severe symptoms that are inadequately controlled by medications or for whom medications cause undesirable side effects. AIT is also indicated for patients who wish to reduce their long-term reliance on pharmacotherapy. For asthma, AIT is most appropriate for those with mild-to-moderate persistent asthma where an allergic component is confirmed. Children as young as 5 years can be treated with SCIT, while SLIT is approved for children age 5 and older for certain allergens.

Contraindications and Risks

Absolute contraindications include severe, poorly controlled asthma (FEV1 < 70% predicted), active autoimmune diseases, malignancies, and the use of beta-blockers (which can worsen anaphylaxis). Relative contraindications include pregnancy (AIT should not be initiated during pregnancy but can be continued if well-tolerated) and the presence of other severe systemic diseases. Side effects of SCIT include local reactions (swelling, redness at injection site) and systemic reactions (urticaria, rhinitis, asthma, anaphylaxis) in a small percentage of patients. SLIT has a lower risk of systemic reactions, with common side effects limited to local oral itching, lip swelling, and throat irritation, which usually resolve with continued use. All AIT must be prescribed and supervised by a board-certified allergist to ensure safety and efficacy.

Comparative Effectiveness: Immunotherapy vs. Standard Medications

While antihistamines, intranasal corticosteroids, and leukotriene receptor antagonists are effective for symptomatic relief, they do not modify the underlying allergic disease. In contrast, AIT is the only treatment that can induce long-term tolerance, with clinical benefits persisting for years after discontinuation. Head-to-head studies comparing AIT to pharmacotherapy are limited, but existing evidence suggests that AIT provides superior long-term outcomes in terms of symptom control, medication reduction, and quality of life. For example, a 2018 randomized trial found that grass pollen SLIT led to significantly greater improvements in rhinoconjunctivitis quality-of-life scores and asthma control compared to standard medications after three years of treatment. Moreover, health-economic analyses indicate that AIT is cost-effective over the long term due to reduced medication use, fewer emergency visits, and improved productivity.

The Future of Allergy Immunotherapy

Personalized Allergy Vaccines

Advances in molecular allergology are paving the way for component-resolved diagnostics and patient-tailored AIT. Instead of using whole allergen extracts—which vary in composition and potency—future therapies may utilize recombinant allergens or allergen peptides that target specific IgE-binding epitopes. This approach could improve safety by reducing the risk of IgE-mediated cross-linking and enhance efficacy by precisely adjusting the antigen profile. Clinical trials of synthetic peptide immunotherapy for cat and grass allergies have shown promise in reducing symptoms with fewer doses. Additionally, virus-like particles and other adjuvants are being explored to boost the tolerogenic response.

Biologics and Combination Therapies

Biologic agents that target the type 2 inflammatory pathway, such as omalizumab (anti-IgE), mepolizumab (anti-IL-5), and dupilumab (anti-IL-4Rα), have revolutionized severe asthma treatment. Emerging evidence suggests that combining biologics with AIT may offer synergistic benefits. Omalizumab, in particular, has been studied as a pre-treatment to reduce the risk of systemic reactions during SCIT build-up, allowing more rapid dose escalation. Furthermore, AIT may help facilitate biologic tapering in patients with severe allergic asthma. Future protocols may integrate AIT as a disease-modifying intervention alongside biologics for refractory cases, personalizing both the trigger (allergen) and the immune pathway (cytokine) simultaneously.

Practical Considerations and Treatment Protocols

AIT requires a substantial time commitment from patients. SCIT involves weekly injections during a 4–6 month build-up phase, followed by maintenance injections every 3–4 weeks for 3–5 years. SLIT requires daily self-administration, with the first dose given under medical supervision. Patients must adhere to the prescribed regimen to achieve optimal outcomes. Monitoring includes periodic assessments of symptom control, spirometry (for asthma patients), and, in some cases, spirometry before each injection. Systemic reactions are rare but must be managed promptly; all AIT providers must have emergency equipment and staff trained in anaphylaxis management. Contraindications should be reassessed at each visit. The decision to discontinue AIT is individualized; many experts recommend a minimum of 3 years of treatment to achieve sustained tolerance. If symptoms recur after discontinuation, a second course of AIT may be considered.

Conclusion

Immunotherapy stands as a unique and powerful tool in the armamentarium against allergic diseases and asthma. By addressing the root cause of allergic inflammation, AIT offers the potential for long-lasting remission, improved quality of life, and reduced healthcare burden. With over a century of clinical use and continuous refinement—from SCIT to SLIT to future personalized vaccines—immunotherapy remains a cornerstone of treatment for appropriately selected patients. Ongoing research into biomarkers, recombinant allergens, and combination strategies promises to further enhance its efficacy and accessibility. For patients struggling with allergic rhinitis or asthma that is not fully controlled by medications, consultation with an allergist to evaluate the suitability of immunotherapy is a proactive step toward lasting relief and better respiratory health.