An allergy is essentially an overreaction by the immune system to a substance that is otherwise harmless, which is known as an allergen. While the immune response in an allergy shares some characteristics with the immune response during an infection, the body’s mechanism for generating a fever is distinct from the typical allergic pathway. Understanding this difference helps clarify why a standard allergy usually does not result in an elevated temperature.
Allergic Reactions and the Absence of Fever
Typical, immediate allergic reactions, such as seasonal hay fever, hives, or mild food sensitivities, generally do not cause a fever. This type of reaction is classified as a Type I hypersensitivity, primarily mediated by immunoglobulin E (IgE) antibodies. IgE antibodies are pre-bound to the surface of mast cells, which are immune cells located in tissues throughout the body.
When an allergen is encountered, it cross-links two IgE antibodies on the mast cell surface, triggering the rapid release of stored chemical mediators, most notably histamine. Histamine and other mediators cause the familiar symptoms of allergy, like swelling, itching, and increased mucus production. This localized inflammation, while uncomfortable, does not typically prompt the widespread systemic signaling required to reset the body’s thermal set point. The chemical cascade involved in this immediate response lacks the potent pyrogenic molecules that drive a true fever.
What Causes Fever in the Body
Fever, or pyrexia, is a tightly regulated physiological response defined by a controlled increase in the body’s core temperature set point. This process is orchestrated by the hypothalamus, a region in the brain that functions as the body’s thermostat. The fever response is usually initiated by pyrogens, which are substances that signal the presence of a threat, typically an infection.
These pyrogens can be molecules from external sources, such as bacteria or viruses, or they can be internal (endogenous) pyrogens released by the body’s own immune cells. Endogenous pyrogens include specific signaling proteins called cytokines, such as interleukin-1, interleukin-6, and tumor necrosis factor-alpha. These cytokines travel through the bloodstream to the hypothalamus, where they stimulate the production of prostaglandin E2 (PGE2). PGE2 then acts on thermosensitive neurons, raising the thermoregulatory set point, which causes the body to generate and conserve heat.
When Allergy Symptoms and Fever Overlap
While a typical IgE-mediated allergic reaction does not generate a fever, there are several situations where a person may experience both allergy-like symptoms and an elevated temperature. The most common scenario involves a secondary infection. Chronic allergic inflammation, such as that seen in allergic rhinitis, can cause prolonged swelling and congestion in the nasal passages and sinuses.
This allergic inflammation can block the normal drainage pathways of the sinuses, leading to a buildup of mucus. The stagnant mucus creates a favorable environment for bacteria or viruses to multiply, resulting in a secondary bacterial or viral infection, like sinusitis or otitis media. In these cases, the fever is caused by the pyrogens released in response to the infection, not the underlying allergy. The allergy is merely a predisposing factor that compromised the tissue defenses.
Severe Systemic Hypersensitivity Reactions
There are also rare, severe forms of systemic hypersensitivity reactions that fundamentally differ from common allergies and do include fever as a primary symptom. These are non-IgE-mediated reactions that involve complex immune mechanisms and are often triggered by medications. One example is Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS syndrome). DRESS syndrome is a delayed, severe reaction characterized by a rash, fever, lymphadenopathy, and internal organ involvement, which can appear several weeks after starting a new medication.
Another such reaction is serum sickness, or a serum sickness-like reaction, which is a Type III hypersensitivity. This condition is also typically drug-induced and presents with fever, rash, and joint pain, usually appearing one to three weeks after exposure. These systemic reactions involve a widespread inflammatory response, including the release of pyrogenic cytokines, which is why fever is a characteristic feature. These severe drug reactions are medical emergencies and require immediate cessation of the offending medication and medical attention. If a fever accompanies allergy-like symptoms, especially if they are severe or systemic, consulting a healthcare professional is necessary to rule out infection or a dangerous hypersensitivity reaction.