Allergy shots contain tiny amounts of the substances you’re allergic to, mixed with a few inactive ingredients that keep the solution stable and sterile. The active ingredients are allergen extracts, which are complex mixtures of proteins pulled from natural sources like pollen, mold, animal dander, insect venom, and dust mites. The inactive ingredients typically include saline, glycerin, a preservative called phenol, and sometimes a stabilizer like human serum albumin.
The Active Ingredients: Allergen Extracts
The core of every allergy shot is one or more allergen extracts. These are made by soaking raw allergenic material (grass pollen, cat dander, mold spores, and so on) in a liquid solution to pull out the proteins. The result is a complex mixture containing allergenic and nonallergenic substances, including proteins, glycoproteins, polysaccharides, lipids, and various other biological compounds. The proteins are the key players. They’re the molecules your immune system has mistakenly flagged as threats, and they’re what make the shot work.
The FDA has approved injectable allergen extracts from a wide range of natural sources: pollens (grass, tree, weed), molds, insect venoms (bee, wasp, hornet), animal hair and dander, and dust mites. Your allergist selects which extracts go into your vial based on your skin test or blood test results, so each person’s shot is essentially a custom blend tailored to their specific sensitivities.
Because these extracts come from biological sources rather than being synthesized in a lab, batch-to-batch consistency depends on careful quality control of the protein content. Potency is measured in units like Bioequivalent Allergy Units (BAU) or Protein Nitrogen Units (PNU), which help ensure that the dose you receive matches what’s expected to produce a therapeutic response.
Inactive Ingredients in the Vial
Beyond the allergens themselves, the liquid in your shot contains several supporting ingredients:
- Sodium chloride (saline) makes the solution isotonic, meaning it matches the salt concentration of your body so the injection doesn’t sting more than necessary.
- Phenol at a concentration of about 0.4% acts as a preservative, preventing bacterial growth in the vial between uses.
- Glycerin helps stabilize the allergen proteins so they don’t break down over time. Some extracts contain up to 50% glycerin by volume, particularly those used for skin testing, though treatment vials typically contain less.
- Sodium bicarbonate serves as a buffer, keeping the solution’s pH in a range that preserves the allergens and remains comfortable for injection.
- Human serum albumin (HSA) is sometimes added as a stabilizing agent. Allergen proteins have varying stabilities in liquid form, and HSA helps prevent them from degrading or sticking to the walls of the vial.
The exact combination of inactive ingredients depends on the type of allergen. Pollen extracts are typically prepared in a phenol-preserved sodium bicarbonate solution. Mold extracts use phenol-preserved saline. Food and animal dander extracts may be prepared in either phenol-preserved saline or glycerin.
How These Ingredients Retrain Your Immune System
The goal of injecting these allergen proteins is to shift your immune system’s response from overreaction to tolerance. When you’re allergic to something, your body produces a type of antibody called IgE that triggers mast cells and basophils to release histamine and other inflammatory chemicals. That cascade is what causes sneezing, itching, swelling, and congestion.
Repeated exposure to small, controlled doses of the allergen prompts your immune system to produce a different class of antibodies, known as blocking antibodies, that intercept the allergen before it can trigger the IgE pathway. In the first year of treatment, IgG1 antibodies do most of this blocking work. After about a year, IgG4 antibodies take over as the dominant protectors in most people. These blocking antibodies essentially get between the allergen and your allergy-triggering cells, reducing the release of inflammatory chemicals and, over time, reducing your symptoms.
How the Dose Changes Over Time
Allergy shots follow a two-phase schedule, and the difference between the phases is the concentration and frequency of what’s being injected.
During the buildup phase, which typically lasts 3 to 6 months, you receive shots one to three times per week. Each injection contains a slightly higher dose of allergen extract than the last, gradually training your immune system to tolerate more without reacting. Some allergists offer an accelerated buildup schedule where you receive several increasing doses in a single visit, shortening this phase considerably.
Once you reach your target dose, you enter the maintenance phase. The allergen concentration in each shot stays the same, but visits drop to about once a month. This phase generally continues for 3 to 5 years, sometimes longer. The extended timeline is what allows the immune changes to become durable, so the benefits persist even after you stop getting shots.
Why Shots Are Given in a Medical Office
Because allergy shots contain the very substances your body is primed to overreact to, there’s a small but real risk of an allergic reaction to the injection itself. Most reactions are mild and local: redness, swelling, or itching at the injection site. Systemic reactions, including rare cases of anaphylaxis, are possible.
This is why you’re asked to wait in the office for at least 15 to 30 minutes after each shot. The clinic keeps epinephrine and other emergency medications on hand. If you carry an epinephrine autoinjector for other allergies, bringing it to your appointment is a reasonable precaution. The risk of a serious reaction is highest during the buildup phase, when your dose is increasing, and drops once you reach a stable maintenance dose.