Allergies, also known as allergic rhinitis or the misleadingly named “hay fever,” are an immune response to harmless environmental substances like pollen, pet dander, or dust mites. When encountering an allergen, the immune system overreacts, releasing chemicals like histamine that cause symptoms such as sneezing, congestion, a runny nose, and itchy eyes. The medical consensus is that allergies do not cause a true fever. The presence of an elevated body temperature points to a different process, typically a response to an infection.
How the Body Creates a True Fever
A true fever, or pyrexia, is a controlled physiological event involving a systemic adjustment of the body’s internal thermostat, the hypothalamus. This response is triggered by pyrogens, fever-inducing substances released during an infection, often from bacteria or viruses. Pyrogens signal immune cells to release specific signaling proteins, known as cytokines, such as Interleukin-1 and Interleukin-6.
These cytokines travel to the brain, stimulating the production of prostaglandin E2 (PGE2) in the anterior hypothalamus. PGE2 effectively “resets” the hypothalamic set-point to a higher temperature, typically 100.4°F (38°C) or higher, which defines a fever. The body then initiates heat-producing mechanisms, such as shivering, to raise the core temperature to this new set-point. Allergies involve localized inflammation and histamine release, a pathway that does not trigger the systemic pyrogens required for this hypothalamic reset.
The Allergy Symptoms That Mimic Fever
Although allergies do not initiate the physiological cascade that results in fever, they can create symptoms that make a person feel “feverish” or unwell. Severe allergic reactions, particularly allergic rhinitis, cause inflammation and pressure in the sinus cavities. This constant inflammatory state and resulting nasal congestion are exhausting, contributing to fatigue and malaise often associated with fever-causing illnesses.
Sinus pressure and inflammation can also trigger headaches and facial pain, which may be misinterpreted as the body aches accompanying a cold or the flu. The ongoing immune response and dealing with chronic symptoms can lead to a subjective feeling of warmth or “hot flashes.” This sensation is not the same as the systemic core temperature elevation of a true fever. In rare instances of severe inflammation, a low-grade temperature elevation might occur, but it remains below the 100.4°F threshold that defines pyrexia.
Using Fever as a Diagnostic Tool
The presence or absence of a true fever is the most reliable way to differentiate simple allergies from an infectious illness. A measured temperature of 100.4°F (38°C) or higher strongly indicates the body is fighting a bacterial or viral infection, not just reacting to an allergen. When symptoms like congestion, runny nose, and cough appear alongside a fever, the cause is likely a cold, the flu, or another viral illness.
Allergies begin immediately upon exposure to an allergen and lack an incubation period, unlike viral infections which involve a slow onset of symptoms. However, persistent, severe allergy symptoms can lead to complications that do cause a fever, most commonly a secondary bacterial sinus infection. The inflammation and mucus buildup caused by allergies create an environment where bacteria flourish, leading to an infection. If allergy-like symptoms persist and a thermometer confirms a true fever, seek medical advice to check for an underlying infection.