Allergy shots, formally known as subcutaneous immunotherapy (SCIT), are a long-term treatment designed to gradually desensitize the body to specific allergens. This process involves regular injections of small, increasing amounts of the substances that trigger allergic reactions. Patients often ask about potential side effects, particularly whether the shots can cause a fever.
Understanding Side Effects and Systemic Reactions
Yes, an allergy shot can cause a fever, but this is an uncommon systemic reaction. Reactions to SCIT are divided into two main groups. The most frequent is a local reaction, involving swelling, redness, or itching confined to the injection site.
Systemic reactions affect the body as a whole and occur less frequently than local ones. These reactions range from mild symptoms, such as sneezing, nasal congestion, or hives, to more severe events. A low-grade, transient fever is sometimes reported as a mild systemic symptom, often accompanying flu-like complaints. Any systemic symptoms, even if minor, should be reported to the allergist.
Why the Immune System Causes Fever
The allergy shot intentionally activates the immune system by exposing it to the allergen in a controlled manner. The shot is designed to re-educate the immune response. This exposure causes immune cells to recognize the substance and initiate a defense mechanism.
The immune system responds by releasing specific signaling proteins called inflammatory mediators, such as cytokines. These mediators, also known as endogenous pyrogens, travel through the bloodstream to the hypothalamus, which acts as the body’s thermostat. Their presence temporarily raises the body’s temperature set point, resulting in a fever. Since the allergen dose is carefully controlled, this inflammatory response is usually minimal and short-lived.
Identifying Serious Symptoms and Next Steps
A mild, post-shot fever can usually be managed with rest and over-the-counter fever reducers like acetaminophen or ibuprofen, following provider guidance. Any systemic reaction, including fever, must be communicated to the allergist to ensure the safety and proper dosing of future injections. Patients must remain for observation in the clinic for at least 20 to 30 minutes after receiving a shot, as most serious reactions occur during this time.
Recognizing Anaphylaxis
It is important to differentiate a mild systemic reaction from a severe one, known as anaphylaxis, which requires immediate medical attention. Signs of anaphylaxis include:
- Difficulty breathing or wheezing.
- Throat tightness.
- A sudden drop in blood pressure.
- Widespread hives.
A high fever (above 101°F) or a persistent fever should also be promptly reported, as it may indicate a separate illness or a more significant reaction. Individuals receiving SCIT should carry an epinephrine auto-injector and know how to use it in case of a severe reaction after leaving the clinic.