Allergy shots (subcutaneous immunotherapy) are a long-term treatment involving regular injections of small, gradually increasing amounts of specific allergens. The goal is to induce desensitization, effectively training the immune system to tolerate the allergen and reduce symptom severity over time. While this treatment is generally safe and highly effective for conditions like allergic rhinitis, asthma, and insect venom allergies, it introduces the very agents that can cause a reaction. Understanding the difference between expected minor side effects and signs of a serious allergic event is important for anyone undergoing this process.
Expected Local Reactions
The most common reactions following an allergy shot are mild and confined to the injection site, typically the upper arm. These localized symptoms are expected and reflect the body’s temporary response to the injected allergen extract. Within minutes to a few hours, patients may notice redness, warmth, swelling, or itching at the site. This reaction often presents as a small bump or induration that usually resolves within a day or two.
A reaction is considered a large local reaction if the swelling or redness is the size of a fifty-cent piece or larger and lasts for more than 24 hours. These larger, yet still localized, responses do not require emergency care but should be reported to the administering clinic. Applying a cold compress or using an over-the-counter oral antihistamine can effectively manage discomfort and reduce swelling. If local reactions occur frequently or become progressively larger, the allergist may slightly adjust the subsequent dosing schedule.
Warning Signs of Systemic Reactions
The primary concern with allergy shots is the rare possibility of a systemic reaction, which affects multiple body systems away from the injection site. Systemic reactions occur when the allergen spreads quickly through the bloodstream and require immediate medical attention. These reactions can range from mild generalized symptoms, such as hives not near the injection site, sneezing, or nasal congestion, to severe, life-threatening anaphylaxis.
Anaphylaxis is the most serious form of reaction and often develops rapidly, typically within 30 minutes of the injection. Respiratory signs include difficulty breathing, wheezing, persistent coughing, or a sensation of tightness or swelling in the throat. These symptoms signal a dangerous narrowing of the airways. Cardiovascular and neurological symptoms are also serious warning signs, indicating reduced blood pressure and oxygen flow.
Patients may experience sudden dizziness, a rapid or weak pulse, or feel faint and collapse. Gastrointestinal symptoms like severe vomiting or abdominal pain can also occur, especially in young children. Any widespread skin reaction, such as generalized flushing or hives covering large areas of the body, should be treated as a serious systemic event requiring immediate action.
Immediate Response Protocol
The immediate response protocol is standardized and time-sensitive due to the potential for rapid systemic reactions. If you are still at the clinic and experience any symptoms beyond mild local irritation, you must alert the medical staff immediately. Clinics must have trained personnel and emergency medications available to manage anaphylaxis. Staff will typically administer aqueous epinephrine, which acts quickly to reverse the dangerous effects of a systemic reaction.
If a systemic reaction occurs after leaving the clinic, the action must be swift and decisive. Patients at risk for severe reactions are usually prescribed an epinephrine auto-injector, commonly known as an EpiPen. The auto-injector should be used immediately by injecting the dose into the outer mid-thigh, even through clothing. The priority is to administer this life-saving medication first, followed by calling emergency services (911 or local equivalent).
After using the auto-injector, lie flat with your legs elevated unless you are struggling to breathe or vomiting. Epinephrine is fast-acting, but its effects can wear off, potentially leading to a second, or biphasic, reaction. Therefore, all patients who experience a systemic reaction must be transported to an emergency facility for medical observation, often lasting at least four hours.
Minimizing Reaction Risk
Proactive measures can significantly reduce the likelihood of experiencing a severe reaction to immunotherapy. The mandatory observation period requires patients to remain in the clinic for 30 minutes following every injection. This window allows medical staff to monitor for the majority of immediate systemic reactions, which typically occur within this timeframe.
Health Status and Activity
It is important to communicate any changes in your health status to your provider before receiving a shot. You should not receive an injection if you are ill with a fever, have poorly controlled asthma, or are experiencing a flare-up of hives. Strenuous physical activity, such as intense exercise or taking a hot bath, should be avoided for at least two hours after the shot. Increased blood flow from these activities can accelerate the spread of the allergen through the body, potentially increasing the risk of a systemic event.
Medications
Patients must ensure their physician is aware of all medications they are taking, particularly beta-blockers. Beta-blockers, often used for high blood pressure or heart conditions, can interfere with the effectiveness of emergency epinephrine, which is the standard treatment for anaphylaxis. An allergist may also recommend taking a prescribed oral antihistamine before the injection to help mitigate the severity of a local reaction.