Allergic reactions do not directly cause a fever. However, fever and allergy-like symptoms can appear together, often indicating a separate underlying issue or complex immune response.
The Allergic Response Explained
An allergic reaction occurs when the immune system mistakenly identifies a harmless substance, called an allergen, as a threat. Upon exposure, the body produces specific immunoglobulin E (IgE) antibodies. These IgE antibodies attach to mast cells, immune cells abundant in tissues like the skin, airways, and digestive tract.
When the allergen is encountered again, it binds to the IgE antibodies on the mast cell surface. This binding triggers the mast cells to release various chemical mediators, including histamine. Histamine causes common allergy symptoms like itching, swelling, hives, sneezing, and a runny nose. These localized reactions do not usually initiate the systemic inflammatory response that results in a fever.
Fevers An Immune System Perspective
A fever is an elevated body temperature, typically 100.4°F (38°C) or higher, serving as a protective mechanism. It is a regulated increase in the body’s thermostat set point in the hypothalamus. This elevation creates an unfavorable environment for pathogens like bacteria and viruses, inhibiting their multiplication.
Fevers are triggered by pyrogens, which can be external (from microbes) or internal (signaling molecules released by immune cells during infection or inflammation). Pyrogens travel to the hypothalamus, signaling it to raise body temperature. This process differs from immediate, localized allergic reactions.
When Fever and Allergic Reactions Coexist
While allergic reactions do not cause fever, specific situations can present with both fever and allergy-like symptoms. Some drug reactions, for instance, can lead to both skin manifestations and fever. Conditions like Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS) syndrome are medication reactions causing high fever, widespread rash, and internal organ involvement. Similarly, serum sickness-like reactions, also drug-induced, can present with fever and joint pain, distinct from a typical histamine-mediated allergy.
Another common scenario involves secondary infections. Allergic reactions, like asthma exacerbations or extensive eczema, can compromise the body’s defenses, increasing susceptibility to bacterial or viral infections. Chronic inflammation from allergic rhinitis or asthma can lead to sinus infections, which are caused by bacteria or viruses and induce fever. In these cases, the fever is a symptom of the infection, not the allergic reaction itself.
Systemic inflammatory conditions, not strictly allergic reactions, can also manifest with allergy-like symptoms and fever. Autoimmune diseases, for example, involve the immune system attacking the body’s own tissues, causing widespread inflammation, fever, and other symptoms resembling allergic responses.
Distinguishing Symptoms and When to Seek Help
Distinguishing between allergic reactions, infections, and other conditions is important for appropriate care. Typical allergy symptoms include sneezing, clear nasal discharge, itchy eyes, skin rashes like hives, and swelling, usually without a fever. These symptoms often appear quickly after allergen exposure and may persist as long as exposure continues.
In contrast, symptoms of an infection, such as a viral illness, commonly include fever, body aches, fatigue, and sometimes a cough or sore throat. Sinus infections, which can sometimes follow allergies, often present with thick, discolored mucus, facial pain, and pressure, along with fever. If you experience a fever alongside allergy-like symptoms, consult a healthcare professional.
Seek immediate medical attention if symptoms are severe, such as difficulty breathing, swelling of the throat or tongue, widespread rash, or a high fever (103°F or 39.4°C or higher). If a fever lasts longer than three days, or if it is accompanied by severe headache, stiff neck, mental confusion, or persistent vomiting, medical evaluation is necessary.