An allergy is a hypersensitivity reaction where the immune system overreacts to a typically harmless substance, such as pollen or pet dander. This immune activation creates symptoms that mimic a true illness, leading many to wonder if allergies can cause chills or a fever. While a pure allergic reaction rarely causes a true fever, the powerful chemicals released by the body can create the sensation of feeling feverish or having chills. Distinguishing between a feeling of illness and an actual infection is important for determining the correct course of action.
The Allergic Response and Sensation of Chills
When the body encounters an allergen, the immune system initiates a defense mechanism involving the release of various inflammatory mediators. Mast cells and other immune cells degranulate, quickly releasing chemicals like histamine, which causes familiar symptoms such as sneezing, itching, and congestion. This initial phase of the allergic response also involves the production of small signaling proteins called cytokines, such as Interleukin-6 (IL-6) and Tumor Necrosis Factor-alpha (TNF-α).
These cytokines are the same chemical messengers the body uses during a true infection to signal a widespread immune response. They travel through the bloodstream and can cause systemic symptoms like generalized malaise, body aches, and fatigue. When these pro-inflammatory cytokines are released, they affect the body’s thermoregulation. This leads to the subjective sensation of feeling cold or having chills, even though the actual core body temperature remains normal. This feeling of being “run down” or having a feverish chill is a side effect of systemic immune activation, not a sign that the body is attempting to fight an infection.
The intensity of this systemic feeling often correlates with the severity of allergic inflammation in the nose and sinuses. A strong allergic reaction can cause significant inflammation and swelling, leading to headaches and facial pressure. This intense localized inflammation, combined with circulating cytokines, contributes to overall discomfort. This discomfort is easily mistaken for the onset of a cold or the flu.
Secondary Infections Triggered by Allergies
While allergies themselves do not directly cause a fever, they can set the stage for a secondary infection that does. Persistent allergic inflammation, particularly in the upper respiratory tract, can lead to severe nasal and sinus congestion. This swelling blocks the normal drainage pathways of the sinuses, causing mucus to become trapped.
The warm, moist, and stagnant environment created by trapped mucus is an ideal breeding ground for bacteria or viruses. When a bacterial infection takes hold in the sinuses, it is known as bacterial sinusitis. This type of infection causes the immune system to release pyrogens, specific chemicals that reset the body’s internal thermostat. This leads to a true, measurable elevation in core body temperature.
In this scenario, the fever and accompanying chills are symptoms of the bacterial or viral infection, not the allergy itself. The allergy acts as a predisposing factor by impairing the body’s natural clearance mechanism. This distinction is important because a secondary bacterial infection often requires different medical intervention, such as antibiotics, which are ineffective against the underlying allergy.
Differentiating Allergy Symptoms from True Fevers
The most reliable way to determine if symptoms are due to a pure allergy or an infection is to use a thermometer. A true fever is medically defined as a core body temperature of 100.4°F (38°C) or higher. This temperature elevation is not a typical symptom of an uncomplicated allergy. The presence of a temperature at or above this threshold strongly suggests a co-existing infection.
Beyond temperature, several other symptoms can help distinguish the cause of the illness. Allergy symptoms are characterized by significant itching, which may affect the eyes, nose, throat, and roof of the mouth. The nasal discharge is usually thin, clear, and watery. Symptoms often follow a predictable pattern related to exposure to specific seasonal or environmental triggers.
Conversely, symptoms that point toward a true infection include a lack of itching and the presence of thick, discolored nasal discharge, often yellow or green. Infections also commonly cause a sore throat, muscle aches, and general body pain not characteristic of a typical allergic reaction. If a fever is present and accompanied by severe facial pressure or symptoms lasting longer than ten days, a secondary infection is likely and warrants medical evaluation.
Treatment and Relief Strategies
Managing the sensation of chills caused by an allergy involves treating the underlying inflammatory response. Over-the-counter and prescription antihistamines are often the first line of defense. They work to block the effects of histamine release, reducing symptoms like itching, sneezing, and the systemic feeling of malaise. Nasal corticosteroid sprays reduce inflammation directly in the nasal passages, helping to alleviate congestion and pressure.
Decongestants help shrink swollen blood vessels in the nose, improving breathing and sinus drainage, which reduces the risk of secondary infections. For chronic or severe allergies, a healthcare provider may recommend immunotherapy, such as allergy shots, to desensitize the immune system to the allergen over time. If a true fever is measured, it is important to consult a medical professional. This indicates an infection that may require specific treatment, such as antibiotics for a confirmed bacterial sinus infection, or over-the-counter pain relievers to manage the fever.