A fever, an elevation in body temperature above the typical range, is a natural defense mechanism. This organized physiological response actively combats infection, forming a key part of the immune system’s overall response to invading pathogens.
The Body’s Heat-Generating Process
The process of generating a fever begins when substances known as pyrogens enter the bloodstream. These pyrogens can originate from external sources, such as bacterial toxins, or be produced internally by immune cells in response to an infection. Once detected, these pyrogens signal the hypothalamus, a region in the brain often referred to as the body’s thermostat, to reset the normal temperature set point to a higher level.
To reach this new, elevated set point, the body initiates several physiological responses. Peripheral vasoconstriction occurs, narrowing blood vessels near the skin’s surface and shunting blood towards the core to conserve heat. Shivering, which involves rapid muscle contractions, generates additional heat, contributing to the rise in core body temperature. These coordinated actions continue until the body’s temperature matches the new, higher setting mandated by the hypothalamus.
Direct Effects on Invading Microbes
Elevated temperatures associated with fever directly impede the proliferation of many bacteria and viruses. Most pathogens have an optimal temperature range for growth and replication, often aligning with the body’s normal temperature. A higher temperature disrupts their metabolic processes, making it more challenging for them to multiply.
Beyond inhibiting replication, fever also makes the environment less hospitable for microbes by affecting nutrient availability. For instance, the body may reduce the availability of iron, a nutrient many pathogens require to thrive, limiting their growth. Elevated temperatures can also destabilize viral RNA polymerase, essential for viral replication, or interfere with viral entry into host cells. While fever temperatures may not directly “cook” pathogens, these changes hinder their ability to establish and spread infection.
Boosting the Immune System’s Arsenal
Fever enhances the immune system’s response, strengthening the body’s ability to fight off invaders. Elevated temperatures increase the activity and proliferation of white blood cells, the body’s primary defense cells. This includes neutrophils, which engulf pathogens, and lymphocytes, crucial for targeted immune responses. T helper cells, a type of lymphocyte, produce more signaling molecules called cytokines at fever temperatures.
The increased temperature also promotes the accelerated production and release of immune molecules, such as cytokines like interferons and interleukins. These molecules act as messengers, coordinating immune responses and signaling other immune cells to join the fight. Fever improves the delivery of immune cells to infection sites by altering surface proteins on lymphocytes, enhancing their travel through blood vessels and migration into infected tissues. This enhanced mobility ensures immune cells reach the battleground more quickly and in greater numbers.
When Fever Requires Attention
While fever is a beneficial response, certain circumstances warrant medical attention. A very high fever, generally 103°F (39.4°C) or higher in adults, or prolonged fever, lasting more than three days, should prompt a call to a healthcare provider. For infants younger than three months, a rectal temperature of 100.4°F (38°C) or higher always requires immediate medical evaluation.
The presence of other concerning symptoms alongside a fever indicates a need for professional assessment. These symptoms can include a severe headache, a stiff neck, a rash, unusual sensitivity to light, persistent vomiting, difficulty breathing, confusion, or seizures. These signs may suggest a more serious underlying condition that requires prompt diagnosis and treatment.