Allergies are a common immune system overreaction where the body mistakenly identifies a harmless substance, such as pollen or dust, resulting in familiar symptoms like sneezing and congestion. While an allergic reaction can make you feel warm, it does not typically cause a true, systemic fever.
The Allergic Response and Temperature Regulation
The warmth or slight temperature elevation that sometimes accompanies an allergic attack is a direct result of the immune system’s local chemical release. When an allergen is encountered, immune cells called mast cells release potent inflammatory mediators, most notably histamine. Histamine’s action causes small blood vessels to dilate, or widen, in the affected area, such as the nasal passages or the skin.
This localized vasodilation increases blood flow to the tissue, which is one of the classic signs of inflammation known as calor, or heat. The increased blood circulation brings warmth to the area, causing a congested nose to feel hot or a patch of skin with hives to feel feverish to the touch. Other inflammatory chemicals, such as certain cytokines, are also released, which further contribute to this local warming sensation. This process is a localized inflammatory response, not a regulated increase in the body’s core temperature.
Distinguishing Inflammation from True Fever
It is important to understand the difference between the feeling of being warm from inflammation and a true fever, or pyrexia. A true fever is defined by a body temperature of 100.4°F (38°C) or higher, and it represents a regulated change in the body’s internal thermostat. This regulation occurs when molecules called pyrogens, typically produced in response to a bacterial or viral infection, signal the hypothalamus in the brain to raise the body’s temperature set point.
Allergic reactions do not involve the presence of pathogens like bacteria or viruses, so they do not trigger the release of these specific pyrogens. Therefore, the body’s thermostat remains unchanged during an uncomplicated allergic episode. Any temperature reading below 100.4°F is considered a low-grade temperature fluctuation or a subjective feeling of warmth, not an actual fever. The term “hay fever” is misleading because it describes a set of allergic symptoms, not a febrile illness.
Secondary Infections as the Cause of Elevated Temperature
The primary reason an individual with allergies might register a true fever is the development of a secondary infection. Sustained inflammation and mucus production in the nasal passages and sinuses, common with allergic rhinitis, create an ideal environment for microorganisms to thrive. The congestion physically obstructs the normal drainage pathways, allowing trapped mucus to become stagnant.
When the stagnant mucus becomes colonized by bacteria or viruses, a secondary infection such as acute bacterial sinusitis can take hold. This new infection introduces the necessary pathogens that trigger a genuine fever. Other potential secondary infections include middle ear infections (otitis media) or bronchitis, especially if the allergic inflammation extends into the lower respiratory tract.
Indications that a fever is due to a secondary infection rather than a simple allergic reaction include symptoms that change or worsen after several days. These signs often involve thick, colored nasal discharge, facial pain or pressure that is severe or localized, and prolonged fatigue. A fever that persists for more than a few days alongside these symptoms is a strong signal that a bacterial or viral complication has developed. Managing the underlying allergic inflammation is often the first step in preventing these infections.
When to Consult a Healthcare Provider
A true fever that occurs during an allergic period warrants consultation with a healthcare provider, especially if it exceeds certain thresholds. You should seek medical advice if your temperature reaches 104°F (40°C) or higher, or if a fever of any degree lasts longer than three days. For infants under three months of age, a temperature of 100.4°F (38°C) or above should be addressed immediately.
Accompanying severe symptoms also indicate the need for prompt medical attention, even with a lower temperature reading. These signs suggest a potentially serious infection or complication that requires professional diagnosis and treatment beyond typical allergy management.
Symptoms Requiring Immediate Attention
- A stiff neck
- Sudden confusion
- Severe pain in the head or chest
- Any difficulty breathing