Allergen Extracts in Immunotherapy: What Is in Your Allergy Shot Vial

An allergen extract is a solution containing proteins derived from a specific allergen source — grass pollen grains, house dust mite bodies and waste products, cat hair and skin cells (dander), cockroach body parts, Hymenoptera venom sacs, or mold spores and hyphae. Manufacturers extract these proteins using aqueous solutions, then purify, filter, and concentrate them into a stable liquid formulation. The extract captures the major allergen proteins — the specific proteins most patients become sensitized to — along with many minor proteins of varying clinical relevance. The goal is a biologically active preparation that, when injected subcutaneously at appropriate doses, will drive the immune system toward allergen-specific tolerance.

FDA standardization means that the potency of the extract — the amount of biologically active major allergen per milliliter — has been validated using biological or immunological assays and is consistently enforced across manufacturing lots. Only 19 allergen extracts meet this standard. For non-standardized extracts, potency is expressed in Protein Nitrogen Units per milliliter (PNU/mL) or weight-to-volume ratios (e.g., 1:20 w/v), which measure total protein concentration rather than specific allergen content. This means potency can vary significantly between production lots and even between manufacturers of the same allergen. In clinical practice, board-certified allergists account for this by using dose ranges rather than single target doses and by being conservative when transitioning to new vials from a new lot.

Certain allergen extracts contain proteolytic enzymes (proteases) — biological catalysts that break down proteins. House dust mites, Alternaria and other molds, and cockroach extracts all have significant protease activity. When mixed in a single vial with protease-sensitive extracts like grass pollen, ragweed, cat dander, or dog dander, these enzymes gradually degrade the other allergens over days to weeks. The result is that a vial prepared with standard mixing may progressively lose potency for the sensitive allergens while the protease-rich allergens remain active. This means the patient receives lower-than-intended doses of certain allergens. Standard practice is to prepare protease-rich extracts in their own dedicated vial — molds and cockroach together in one vial, inhalant allergens in another (Grier et al., Ann Allergy Asthma Immunol 2007).

Extract shelf life depends significantly on glycerin content and storage temperature. Maintenance concentrate vials containing 50% glycerin (v/v) remain potent for 3-5 years when refrigerated at 35-46°F. Dilute vials (1:10 through 1:10,000 dilutions) that are non-glycerinated lose 50-90% of their potency within 3-6 months at room temperature and can degrade significantly even under refrigeration (Nelson, JACI 2007). Human serum albumin is added to the most dilute vials to prevent allergen adsorption to vial walls — a secondary potency loss mechanism. Patients should never use vials stored improperly (left unrefrigerated for extended periods) and should alert their clinic if a vial has been accidentally left out of refrigeration.

The maintenance dose is the target therapeutic amount of allergen that your treatment is working toward during the build-up phase. For inhalant allergens (pollen, dust mite, pet dander, mold), the AAAAI/ACAAI Practice Parameter targets 5-20 micrograms of the major allergen protein per 0.5 mL injection dose. For FDA-standardized allergens, this translates to specific unit values: 500-2,000 AU for dust mites, 1,000-4,000 BAU for grass pollen and cat dander. For Hymenoptera venom, the target is 100 micrograms per venom species. Patients who do not tolerate build-up to full maintenance may achieve some benefit at lower doses, but dose-dependent efficacy means full maintenance delivery maximizes the clinical response.

The majority of allergen extracts used in US allergy practices come from three FDA-licensed manufacturers: Greer Laboratories (Lenoir, NC), ALK-Abello (Round Rock, TX), and Hollister-Stier (Spokane, WA). Each manufacturer produces a broad range of allergen extracts from standardized and non-standardized sources. Individual allergist practices typically obtain the commercial extract concentrates from these manufacturers and then prepare personalized patient vials in their own compounding area by combining allergens and creating the dilution series. The practice-level preparation step — mixing allergens and creating serial dilutions — is where allergist expertise in extract compatibility and dosing is applied.

The starting dose for allergy shot build-up is typically 1,000-10,000 times more dilute than the target maintenance dose. This extreme dilution serves two purposes: safety and immunological priming. In a sensitized patient, the immune system has pre-formed IgE antibodies against the target allergen. Delivering a full maintenance dose from the start would overwhelm local mast cells and potentially trigger severe systemic reactions. Beginning at highly dilute concentrations allows the immune system to begin early desensitization — mast cell and basophil down-regulation — before significant allergen exposure occurs. Novak et al. (JACI 2012) showed that basophil histamine-2 receptor upregulation, an early desensitization marker, begins within the first 6 hours of build-up, demonstrating that even very low starting doses trigger meaningful immune changes.