An allergy is a misdirected immune response to a harmless substance, such as pollen or dust, which the body treats as a threat. This reaction releases chemicals that cause symptoms like sneezing, congestion, and itching. A fever is an elevated body temperature that signals the body is fighting a true infection from a virus or bacterium. Uncomplicated allergic reactions do not cause a true fever, but the resulting intense inflammation can mimic the discomfort of a mild illness. The connection between allergies and fever is almost always indirect, rooted in distinct biological processes.
The Difference Between Allergic Inflammation and Fever
The mechanisms causing allergic reactions and fevers operate on fundamentally different pathways. An allergic reaction begins when mast cells, sensitized by the antibody Immunoglobulin E (IgE), encounter an allergen. This contact causes the mast cells to rapidly release chemical mediators, most notably histamine, into the local tissues. Histamine drives localized effects like swelling, mucus production, and itching, which are common allergy symptoms.
This process is a localized inflammatory response, typically affecting specific areas like the skin, eyes, or nasal passages. This localized release of histamine does not trigger the systemic release of pyrogens. Pyrogens are molecules, such as Interleukin-1 and Interleukin-6, required to reset the body’s internal thermostat.
A true fever requires pyrogens to travel through the bloodstream to the hypothalamus, the brain’s temperature-regulating center. There, these chemicals trigger the production of prostaglandin E2 (PGE2), which raises the body’s temperature set point. This system-wide elevation is characteristic of an infection, not a standard allergic response. The immune system’s response to a benign allergen does not activate the pyrogen cascade needed to reset the thermostat.
Common Symptoms That Mimic Fever
Even without an elevated temperature, a severe allergic episode can cause a person to feel generally unwell, leading to the subjective impression of a fever. Systemic inflammation and immune activation result in generalized symptoms like malaise, a feeling of discomfort and fatigue. This exhaustion is compounded by sleep disruption often caused by severe congestion, coughing, and postnasal drip.
It is common to experience muscle aches and a general sense of being run down, similar to signs of a viral infection. The term “hay fever” is a misnomer, as allergic rhinitis is not associated with a true fever. The sensation of being flushed or warm may be due to localized inflammation or the effort expended during a prolonged reaction. Only an accurate temperature reading can distinguish this subjective feeling from a medically defined fever.
Secondary Infections Triggered by Allergies
An allergy commonly leads to a fever by creating a physical environment that welcomes a secondary bacterial or viral infection. Chronic allergic rhinitis causes persistent inflammation and swelling of the nasal and sinus passages. This swelling blocks the small openings that allow mucus to drain from the sinuses, leading to fluid buildup.
This stagnant, nutrient-rich mucus becomes an ideal breeding ground for bacteria or viruses. When these trapped microorganisms proliferate, they cause an acute infection, such as bacterial sinusitis. It is this secondary infection, not the allergy itself, that prompts the systemic release of pyrogens, causing the fever.
Chronic congestion can also block the Eustachian tubes, which connect the middle ear to the back of the throat. This blockage prevents proper drainage and ventilation, increasing the risk of otitis media, a middle ear infection. Individuals with allergic asthma are also more susceptible to lower respiratory tract infections like bronchitis or pneumonia. The underlying allergic inflammation compromises the respiratory tract’s natural defenses, allowing pathogens to cause an infection that includes a fever.