An allergy is an immune system overreaction to a harmless substance like pollen or dust, where the body mistakenly identifies it as a threat. This reaction triggers symptoms such as sneezing, itching, and congestion, primarily affecting the respiratory tract. A fever is a systemic elevation of the body’s core temperature, typically reaching 100.4°F (38°C) or higher, often indicating an active infection. While severe allergies can cause discomfort, seasonal allergies alone do not cause a true systemic fever.
Why Allergies Do Not Cause Systemic Fever
Allergic reactions and fevers involve distinct immune pathways, which explains why allergies do not trigger a core temperature change. The allergic response is initiated by Immunoglobulin E (IgE) antibodies, causing mast cells to release inflammatory chemicals, most notably histamine. Histamine primarily causes localized effects, such as swelling in the nasal passages, increased mucus production, and itchy eyes.
This localized inflammation may produce a temporary sensation of warmth or a minor elevation in local tissue temperature. However, this response does not trigger the hypothalamus, the brain’s thermostat, to reset the body’s core temperature upward. The true systemic fever response is instead driven by pyrogens, which are chemical signals released by immune cells, usually in response to a viral or bacterial infection.
Pyrogens travel to the hypothalamus and induce the production of Prostaglandin E2, effectively raising the body’s thermal set point. The body then works to meet this new temperature, causing systemic effects like chills and muscle aches. The mechanism of allergy is focused on immediate, localized defense, fundamentally differing from the systemic, temperature-raising defense of an infection.
Identifying the Symptoms of Secondary Infection
A true fever alongside allergy-like symptoms strongly indicates a secondary infection has developed. Allergy symptoms typically include clear, watery nasal discharge, intense itching of the eyes, nose, or throat, and repetitive sneezing.
When a secondary bacterial or viral infection takes hold, the symptoms shift and intensify. The presence of thick, discolored mucus, often yellow or green, is a significant sign that the issue is no longer just an allergy. Infection-related symptoms, such as fever, body aches, and fatigue, may develop later or persist for a longer duration, often past 10 to 14 days.
Localized pain or pressure, particularly around the cheeks, forehead, or eyes, suggests a sinus infection, which is a common complication. A fever lasting more than three or four days, or symptoms that initially improve before suddenly worsening, should prompt a consultation with a healthcare provider. These signs indicate the immune system has moved from an allergic response to a systemic fight against a pathogen.
The Allergy-Infection Connection
While allergies do not directly cause fever, the inflammation they produce creates conditions that welcome infection, leading to fever indirectly. Allergic reactions cause the tissues lining the nasal passages and sinuses to swell, obstructing the narrow drainage pathways. This swelling prevents the natural flow of mucus, which is normally swept out by tiny hair-like structures called cilia.
When mucus is trapped behind these blocked openings, it pools and stagnates, creating a warm, moist environment. This stagnant fluid becomes an ideal breeding ground for bacteria or viruses, allowing them to multiply and cause a secondary infection, such as acute bacterial sinusitis. The resulting infection triggers the release of pyrogens, initiating the systemic fever.
Common secondary issues include sinus infections (sinusitis) and middle ear infections (otitis media), where inflammation blocks the Eustachian tube. Managing underlying allergies is an important step in preventing the development of these fever-causing secondary infections.