What Are the Ingredients in Allergy Shots?

Allergy shots contain tiny amounts of the substances you are allergic to, mixed with a handful of supporting ingredients that keep the solution stable and help your immune system respond to it. The active ingredient is an allergen extract, a concentrated preparation made from the pollen, mold, dust mite, animal dander, or insect venom that triggers your symptoms. Around that extract sit preservatives, diluents, stabilizers, and sometimes adjuvants, each playing a specific role in making the shot both safe and effective. The exact recipe varies depending on what you are being treated for, where in the world you receive treatment, and which manufacturer produced the extract.

Allergen Extracts Are the Core Ingredient

The whole point of an allergy shot is controlled exposure, so the dominant ingredient is the allergen itself. Manufacturers produce extracts by taking raw source material, such as ragweed pollen, cat dander, or dust mite bodies, and processing it to pull out the proteins that cause allergic reactions. The result is a liquid concentrate containing a complex mix of proteins, glycoproteins, and enzymes derived from the original source.

Your allergist selects which extracts go into your shots based on the results of skin or blood testing. If you are allergic to three types of grass pollen and dust mites, your treatment vial will contain extracts from all of those sources. These individual extracts are combined and then diluted into a series of increasingly concentrated vials, so that over weeks or months your dose gradually increases until you reach a maintenance level your immune system can tolerate.

Not all extracts are created equal. Some are “standardized,” meaning the manufacturer has tested the product against a reference standard to verify its potency and confirm that it will produce a consistent allergic response in sensitive individuals. Others are “non-standardized,” which means they have been characterized less rigorously and can vary more widely in their content of allergenic proteins and in their actual potency from batch to batch. In the United States, only a handful of allergens have FDA-approved standardized extracts, including cat hair and pelt, dust mites, several grass pollens, and short ragweed. Everything else, from tree pollens to mold spores to cockroach, is supplied as a non-standardized extract.

Preservatives and Diluents

Allergen extracts are biological products, and like any protein solution, they degrade over time. The inactive ingredients in your allergy shot exist largely to slow that degradation and keep the proteins intact long enough to do their job.

Phenol is the most common preservative. It is present in small concentrations, typically around 0.4%, and serves as an antimicrobial agent that prevents bacterial growth in the vial. You may notice a faintly medicinal smell when the nurse draws up your injection; that is the phenol. Glycerin is another frequent addition. It acts as both a preservative and a stabilizer, helping allergen proteins maintain their structure in solution. Glycerin-based extracts tend to have a longer shelf life, and concentrates stored at 50% glycerin hold up well for extended periods. The tradeoff is that glycerin can sting a bit at the injection site, especially at higher concentrations.

The base liquid in most allergy shots is normal saline, sometimes with phenol added. When clinics dilute concentrated extracts down to your starting dose, they use these saline-phenol solutions as the vehicle. Some practitioners also add human serum albumin (HSA), a protein naturally found in blood, to the diluent. HSA coats the inside of the glass vial and prevents the allergen proteins from sticking to the walls and losing potency, a problem that becomes more pronounced as solutions get more dilute.¹ You will sometimes see sodium bicarbonate or sodium chloride listed on the extract label as well; these are simple buffering agents that keep the pH in a range where proteins stay stable.

Adjuvants and Why They Matter

An adjuvant is a substance added to boost the immune response to whatever it is paired with. In allergy immunotherapy, adjuvants serve a dual purpose: they enhance your immune system’s ability to build tolerance to the allergen, and they help keep the allergen at the injection site rather than letting it flood into the bloodstream too quickly, which reduces the risk of a systemic reaction.

Aluminum hydroxide is by far the most widely used adjuvant in subcutaneous allergy immunotherapy worldwide.² It works by adsorbing the allergen onto its surface, creating a slow-release depot under the skin. This is the same type of aluminum salt used in many childhood vaccines, and the amounts involved in allergy shots are small. Calcium phosphate performs a similar depot function and is used in some European products as an alternative.³

It is worth noting that not every allergy shot contains an adjuvant. In the United States, most conventional aqueous extract formulations used in allergist offices do not include aluminum hydroxide or any other adjuvant. The extracts are simply mixed, diluted, and injected. Adjuvants are more common in European depot formulations, where they are paired with modified allergens. Sublingual immunotherapy tablets and drops, which dissolve under the tongue rather than being injected, rely on high doses of aqueous allergen extract with no adjuvant at all.³

Other Adjuvants Used in Europe

If you receive allergy immunotherapy in Europe, you may encounter adjuvants that are not available in the US. A European Academy of Allergy and Clinical Immunology position paper identified four adjuvants in marketed products across the continent: aluminum hydroxide, microcrystalline tyrosine (MCT), monophosphoryl lipid A (MPLA), and calcium phosphate.⁴ Aluminum hydroxide remains the most frequently used, but the others occupy meaningful niches.

MCT is a biodegradable amino acid that acts as a depot adjuvant, slowly releasing the allergen from the injection site. Unlike aluminum hydroxide, MCT breaks down completely in the body within about 48 hours, which some researchers consider an advantage for reducing long-term tissue accumulation. MPLA is derived from bacterial lipopolysaccharide and acts as an immunostimulant, actively pushing the immune response toward the kind of tolerance that allergy immunotherapy aims for. Some European products combine MCT and MPLA together with a chemically modified allergen in a single formulation.⁵

These different adjuvants do not all work the same way immunologically. Aluminum hydroxide initially boosts the type of immune response already overactive in allergy (Th2), before eventually supporting tolerance. The other three adjuvants tend to redirect the immune response toward Th1 activity more directly, which may explain why some European products use shorter treatment courses.⁴

The Problem With Mixing Extracts Together

Because most allergy patients react to more than one substance, clinics routinely combine multiple allergen extracts into a single treatment vial. This is convenient but introduces a real complication: some extracts contain enzymes called proteases that chew up the allergenic proteins from other extracts in the same vial, reducing or destroying their potency before the shot ever reaches your arm.

Mold and cockroach extracts are the worst offenders. They are naturally rich in proteases, and when mixed with pollen or dust mite extracts, those proteases can degrade the target allergens significantly. One study found that allergen stability was compromised in certain fungal-insect extract mixtures, meaning the final product no longer delivered the intended dose of those allergens.⁶ The practical solution is to separate incompatible extracts into different vials. Your allergist may prescribe two or even three separate injection vials rather than putting everything into one, specifically to avoid this kind of degradation.⁷

Glycerin can help buffer this problem somewhat, since higher glycerin concentrations tend to slow protease activity. This is one reason why concentrated stock extracts preserved in glycerin are more stable than diluted aqueous formulations. But once extracts are diluted for injection, that protective effect diminishes, and separating high-protease extracts from protease-sensitive ones becomes the more reliable strategy.

Unwanted Components That Tag Along

Because allergen extracts are derived from natural biological sources rather than synthesized in a lab, they inevitably contain more than just the target allergen. Endotoxins, fragments of bacterial cell walls, are a notable example. A study testing 58 standardized allergen vaccines found endotoxin levels ranging from undetectable to 34,000 endotoxin units per milliliter, with enormous variation depending on the source material. Cat pelt extracts carried far more endotoxin than cat hair extracts, and dust mite species differed dramatically from each other. Grass and ragweed pollen extracts contained comparatively less.⁸

These endotoxins are not intentionally added, and they are not the reason the shots work. But their presence is not entirely irrelevant either, because endotoxins stimulate the immune system in ways that could theoretically influence the outcome of treatment, for better or worse. The wide variability in endotoxin content across products from different manufacturers is one more factor that makes lot-to-lot consistency a challenge in this field.

Other non-allergenic proteins, lipids, and carbohydrates from the source material are also present in crude extracts. A ragweed extract, for instance, does not contain only the Amb a 1 protein that causes most ragweed allergy. It contains hundreds of other ragweed-derived molecules. Some of these may be immunologically inert, and some may contribute minor effects. This complexity is a key limitation of extract-based immunotherapy and one of the motivations behind newer molecular approaches.

Allergoids and Chemically Modified Allergens

In several European countries, allergy shots use a modified form of the allergen called an allergoid rather than the native protein. Allergoids are produced by chemically treating the natural extract, typically with formaldehyde or glutaraldehyde, which alters the shape of the protein in a way that reduces its ability to bind to the antibodies (IgE) that trigger allergic reactions. The T-cell recognition sites on the protein, which are what the immune system needs to build tolerance, remain largely intact.⁹

The result is a product that is less likely to cause an immediate allergic reaction at the injection site but can still train the immune system to tolerate the allergen. This allows for faster dose escalation and shorter build-up schedules than conventional aqueous extracts. Allergoids are widely used in Germany, Spain, Italy, and other European countries but are not available in the United States, where regulatory pathways for these products have not been established.¹⁰

How US and European Formulations Differ

If you have received allergy shots in the US and then move to Europe, or vice versa, you will find that the products look quite different. American immunotherapy is dominated by aqueous allergen extracts, custom-mixed by the allergist’s office from commercially available concentrated extracts. The allergist selects which extracts to include, determines the dilution schedule, and mixes the treatment vials on-site. This bespoke approach allows enormous flexibility but also means the final product depends heavily on the skill and knowledge of the person preparing it.

European immunotherapy tends to rely more on pre-manufactured, named products from pharmaceutical companies. Many of these are depot formulations with aluminum hydroxide or another adjuvant already incorporated, and many use allergoids rather than native extracts. The regulatory frameworks differ as well. European products undergo a registration process more similar to conventional pharmaceuticals, while US allergen extracts are regulated as biologics with their own distinct set of requirements. Reference preparations and standardized potency assays are handled differently between the two regions.¹⁰

Neither system is obviously better. The US model offers customization, and the European model offers standardization. But the differences mean that reading about allergy shot ingredients online can be confusing, because advice and product descriptions written for a European audience will mention adjuvants and allergoids that an American patient will never encounter, and American content may describe mixing practices that are unfamiliar in Europe.

Recombinant Allergens and the Future of What Goes in the Vial

One of the persistent frustrations with conventional allergen extracts is that they are messy. They contain hundreds of proteins when only a few are responsible for most of the allergic response. They vary in potency between manufacturers and between batches. And as discussed, their complex composition creates compatibility problems when mixed.

Recombinant allergens aim to solve these problems. Instead of grinding up pollen or mite bodies and extracting everything, researchers use genetically engineered cells to produce only the specific allergenic proteins needed for treatment. The resulting product is chemically defined, consistent from batch to batch, and free of the non-allergenic contaminants found in natural extracts. Clinical studies suggest that recombinant allergen-based vaccines can be developed that are superior to existing extract-based products for treating respiratory, food, and venom allergies.¹¹

Panels of unmodified recombinant allergens have already been shown to effectively replace natural allergen extracts in therapy in clinical trials.¹² Beyond simple recombinant proteins, researchers are also developing hypoallergens (recombinant proteins engineered to have reduced IgE binding, similar in concept to chemically produced allergoids but with more precision) and allergen peptides (short fragments of the protein that stimulate tolerance without triggering an allergic response at all). None of these are yet in routine clinical use for most allergens, but they represent the direction the field is heading. When they arrive, the ingredient list for allergy shots may become dramatically simpler and more precise than today’s complex biological mixtures.

What Venom Shots Contain

Allergy shots for insect venom, used to protect people who have had severe reactions to bee stings, wasp stings, or fire ant bites, deserve a separate mention because their active ingredient is fundamentally different from the pollen and dander extracts described above. Venom immunotherapy uses purified venom collected from the insects themselves. Honeybee venom, for example, is collected using electrical stimulation of the hive, then processed and standardized to a known protein concentration. The supporting ingredients, preservatives, diluents, and sometimes albumin as a stabilizer, are broadly the same as in environmental allergy shots. But the allergen itself is a venom protein rather than an inhaled environmental protein, and the clinical stakes are higher, since venom allergy can cause life-threatening anaphylaxis. This is one area where extract standardization has been well established for decades, and venom immunotherapy is among the most effective forms of allergen-specific treatment available.

Fire ant immunotherapy is a partial exception. While purified venom products exist for flying insects like bees and wasps, fire ant treatment in the US has historically relied on whole-body extract rather than isolated venom, in part because extracting venom from ants in sufficient quantities is impractical. Whole-body extract contains the venom proteins along with a great deal of other ant-derived material, making it a cruder product. Despite this, it remains effective in clinical practice, which tells you something about how forgiving the immune system can be when it comes to the purity of these treatments.