Allergy Immunotherapy: Shots, Drops, and Desensitization

Allergy immunotherapy is a treatment category that modifies the immune system's response to specific allergens rather than simply suppressing symptoms. This page covers the two primary delivery formats — subcutaneous injections and sublingual drops or tablets — alongside the biological mechanisms that drive desensitization, classification boundaries between approved and investigational protocols, and the known tradeoffs that shape clinical decision-making. Understanding immunotherapy's structure is foundational to navigating the broader allergy treatment landscape.

Definition and scope

Allergy immunotherapy (AIT) is a long-term treatment strategy that involves the repeated, graduated administration of an allergen to reduce the severity of allergic reactions over time. The U.S. Food and Drug Administration (FDA) regulates AIT biologics and finished allergen extracts under Title 21 of the Code of Federal Regulations, with allergen extracts classified as biological products subject to licensure under 21 CFR Part 600–680.

The treatment scope encompasses inhalant allergens (pollens, dust mites, mold spores, pet dander), insect venom, and — in specialized protocols — certain food allergens. The regulatory context for allergy treatment in the United States designates allergen extracts used in subcutaneous immunotherapy (SCIT) as prescription biologics, while FDA-approved sublingual immunotherapy (SLIT) tablets exist for a limited allergen set including grass pollen, ragweed, and house dust mite (Dermatophagoides species).

AIT differs categorically from pharmacotherapy: antihistamines and corticosteroids manage symptoms during exposure, while immunotherapy seeks to induce immunological tolerance that persists after treatment ends. The American Academy of Allergy, Asthma & Immunology (AAAAI) recognizes AIT as the only disease-modifying treatment option for IgE-mediated allergic conditions.

Core mechanics or structure

The desensitization mechanism operates through two immunological phases: the build-up phase and the maintenance phase.

During the build-up phase, allergen dose escalates incrementally — typically over 3 to 6 months in conventional protocols — starting well below the threshold that would provoke a systemic reaction. Each dose increment trains immune effector cells to tolerate increasing allergen loads. During maintenance, a stable target dose is administered at regular intervals, typically every 2 to 4 weeks for SCIT, for a total treatment duration of 3 to 5 years per AAAAI guidelines.

At the cellular level, immunotherapy shifts the dominant T-helper cell response from the Th2 pathway (which drives IgE production and allergic inflammation) toward a Th1 and regulatory T-cell (Treg) profile. Key measurable markers of this shift include:

For SCIT, injections are administered in a clinical setting equipped for anaphylaxis management. For FDA-approved SLIT tablets, the first dose is administered under medical supervision; subsequent doses are taken at home. Sublingual drops compounded for individual patients — the most common form of SLIT used in primary care settings — are not FDA-approved and are regulated differently, a distinction covered under sublingual immunotherapy.

Causal relationships or drivers

The biological rationale for immunotherapy rests on the finding that persistent low-dose allergen exposure drives immune deviation away from inflammatory pathways. Three causal chains are well-established in peer-reviewed literature:

1. IgE receptor saturation and downregulation. Repeated sub-threshold allergen exposure gradually saturates high-affinity IgE receptors on mast cells and basophils without triggering degranulation, leading to receptor downregulation over months.

2. Th2-to-Treg shift. Tolerogenic dendritic cells, stimulated by repeated allergen presentation at sub-clinical doses, preferentially activate regulatory T-cells (Tregs) that suppress Th2 effector responses. This is the central mechanism cited by the World Allergy Organization (WAO) in its 2013 SLIT position paper and subsequent updates.

3. IgG4 blocking antibody generation. Elevated IgG4 titers — a consistent finding in successfully treated patients — competitively inhibit IgE-allergen binding, reducing mast cell and basophil activation even when IgE levels remain detectable.

Failure of immunotherapy is causally linked to three primary drivers: insufficient dose delivery, poor adherence to the maintenance schedule (particularly for self-administered SLIT), and selection of patients whose sensitization pattern does not match available extract formulations. Polysensitized patients — those reactive to 5 or more distinct allergen families — present a more complex desensitization challenge because allergen interactions within mixed extracts can reduce individual potency.

Classification boundaries

Immunotherapy protocols separate along four classification axes:

Route of administration: Subcutaneous (injection) vs. sublingual (drops or tablets) vs. oral (food allergen OIT protocols, largely investigational for most foods except peanut).

Regulatory status: FDA-approved finished products (Grastek, Odactra, Ragwitek, Palforzia for peanut) vs. FDA-licensed but compounded allergen extracts used in SCIT vs. non-FDA-approved compounded SLIT drops.

Speed of escalation: Conventional (weeks to months), cluster (grouped doses over fewer visits), and rush immunotherapy (escalation over 1–3 days in hospital settings with higher systemic reaction risk).

Target allergen class: Inhalant allergens, Hymenoptera venom (bee, wasp, yellow jacket — with documented 97% protection rate per AAAAI position statements), and food allergens under oral immunotherapy protocols.

Venom immunotherapy (VIT) occupies a distinct clinical category because anaphylaxis risk from insect stings is life-threatening and VIT efficacy data is among the strongest in the AIT literature. Insect sting allergies and their venom-specific protocols warrant separate consideration from inhalant AIT.

Tradeoffs and tensions

Efficacy vs. convenience. SCIT requires in-office injections with a mandatory 20–30 minute observation period post-injection, creating a significant adherence burden over a 3–5 year treatment course. SLIT tablets allow home administration but are approved for a narrow allergen set. Compounded SLIT drops expand allergen coverage but lack FDA approval, creating regulatory and quality-control uncertainty.

Duration vs. persistence of effect. Published data from randomized controlled trials — including those cited in the Cochrane systematic review on subcutaneous immunotherapy for allergic rhinitis — indicate that 3 years of continuous SCIT produces durable tolerance in a majority of patients, but a subset experiences symptom relapse within 3 years of stopping treatment. Optimal treatment duration remains an active area of investigation.

Systemic reaction risk. SCIT carries a measurable risk of systemic allergic reactions: the AAAAI estimates approximately 1 systemic reaction per 1 million injections administered, with fatal reactions being extremely rare but documented. Rush and cluster protocols carry higher per-visit risk profiles. SLIT has a substantially lower systemic reaction risk but is associated with local oral and throat reactions in 30–40% of users during early treatment, per WAO position documentation.

Pediatric vs. adult considerations. AIT is used in children as young as 5 years in standard clinical practice, with some protocols extending to children aged 2–4 under specialist supervision. Allergies in children involve specific developmental considerations around adherence, dosing, and tolerance durability that differ from adult protocols.

Common misconceptions

Misconception: Immunotherapy cures allergies permanently.
Correction: AIT induces clinical tolerance that persists for years post-treatment in many patients, but tolerance is not universally permanent. Relapse rates vary by allergen, patient age, treatment duration, and maintenance dose achieved. The AAAAI classifies AIT as disease-modifying, not curative.

Misconception: SLIT drops sold online are equivalent to FDA-approved SLIT tablets.
Correction: FDA-approved SLIT tablets (Grastek, Odactra, Ragwitek) undergo standardized potency testing and clinical trial validation. Compounded sublingual drops are not FDA-approved, are not subject to the same potency standardization, and their regulatory status is distinct under 21 CFR compounding provisions.

Misconception: Allergy shots work immediately.
Correction: The build-up phase spans weeks to months, and maximum benefit typically requires 12 months or more of treatment. Symptom reduction during the first season of treatment is partial and incremental.

Misconception: A negative skin test means immunotherapy is not needed.
Correction: Immunotherapy candidacy is determined by the correlation of skin or blood test results with clinical symptom history, not by test result alone. Allergy testing methods provide sensitization data, but clinical judgment integrates multiple factors.

Checklist or steps (non-advisory)

The following describes the sequence of procedural stages in a conventional SCIT protocol as documented by AAAAI practice parameters:

Reference table or matrix

Feature SCIT (Shots) SLIT Tablets (FDA-approved) SLIT Drops (Compounded) Oral Immunotherapy (OIT)
FDA approval status Licensed extracts (21 CFR 600–680); not "approved" as drug products FDA-approved (Grastek, Odactra, Ragwitek, Palforzia) Not FDA-approved Palforzia (peanut) approved; others investigational
Administration site In-office only First dose in-office; remainder at home Typically at home First dose/escalation in-office
Allergen coverage Broad (inhalants, venoms, custom mixes) Narrow (grass, ragweed, dust mite, peanut) Broad (compounded to patient) Food allergens only
Systemic reaction risk ~1 per 1 million injections (AAAAI) Very low Low Low-moderate (dose escalation phase)
Local reaction rate Injection site reactions common Oral/throat reactions 30–40% (WAO) Oral reactions reported Oral/GI reactions common
Treatment duration 3–5 years 3–5 years 3–5 years Ongoing (maintenance indefinite for many)
Potency standardization Variable by allergen; some FDA-standardized FDA-standardized Not standardized FDA-standardized (Palforzia)
Observation requirement 20–30 min post-injection First dose only First dose recommended Each escalation dose

References

The law belongs to the people. Georgia v. Public.Resource.Org, 590 U.S. (2020)