Do neutrophils increase in fever? A closer look at your immune response

September 30, 2025 •

3 min read

A high neutrophil count is a common finding during a fever, especially with a bacterial infection. This physiological response is your body’s immediate call to action, as it mobilizes its most abundant white blood cells to combat invading pathogens. So, do neutrophils increase in fever? The answer is often yes, but the specific cause matters.

Quick Summary

Neutrophils, the most numerous white blood cells, frequently increase during fever in response to infection or inflammation. This neutrophilia is a vital immune reaction, though the cause of the fever dictates the specific blood cell pattern.

Key Points

Normal immune response: Yes, neutrophils typically increase in fever, particularly with bacterial infections, as part of the body's initial immune attack.
Role in defense: Neutrophils are the most abundant white blood cells and act as first responders, engulfing pathogens and releasing antimicrobial compounds.
Enhanced function: The elevated temperature of a fever, known as fever-range hyperthermia, can boost the activity of neutrophils, increasing their efficiency.
Underlying causes: Fever and neutrophilia can be triggered by bacterial infections, inflammation, injury, stress, and certain medications or cancers.
Bacterial vs. viral patterns: High neutrophils with fever often point to a bacterial cause, while viral infections can sometimes lead to lower neutrophil counts (neutropenia).
Clinical interpretation is key: Blood test results must be interpreted alongside other symptoms and the patient’s medical history to form an accurate diagnosis.

What are neutrophils and what is their role?

Neutrophils are a type of white blood cell, also known as leukocytes, and are a key part of the body's initial defense against infection. They are the most abundant leukocyte in the bloodstream, acting as first responders to pathogens like bacteria and fungi. When a threat is detected, neutrophils are sent to the site of infection where they:

Engulf and eliminate microorganisms.
Release potent antimicrobial substances.
Form web-like traps to capture and destroy pathogens.
Help regulate inflammation.

The connection between fever and increased neutrophils

Fever is a common part of the immune response, and research indicates it can enhance the function of immune cells, including neutrophils. This rise in body temperature is triggered by pyrogens, substances released by immune cells in response to infection or injury, which signal the brain to raise the body's temperature.

Fever-range hyperthermia (FRH) can impact neutrophils by:

Increasing their movement to infection sites.
Speeding up immune processes like forming NETs and producing reactive oxygen species.
Stimulating the bone marrow to produce and release more neutrophils, a state called neutrophilia.

Causes of neutrophilia with fever

An elevated neutrophil count with fever, or neutrophilia, is commonly associated with bacterial infections, but other factors can also cause it. These include:

Bacterial Infections: A frequent cause, prompting the body to rapidly produce neutrophils to fight the bacteria.
Inflammatory Conditions: Non-infectious inflammation, such as in autoimmune diseases, can also lead to neutrophilia and fever.
Stress: Physical or emotional stress can increase circulating neutrophils.
Tissue Damage: Injuries can cause fever and increase neutrophils as part of the healing process.
Certain Medications: Some drugs, like corticosteroids, can cause this side effect.
Malignancies: Certain cancers can result in abnormally high neutrophil levels.

Bacterial vs. viral infections: different neutrophil patterns

The pattern of white blood cells can offer clues about the cause of an infection. Bacterial and viral infections often show different patterns:


Feature	Bacterial Infection	Viral Infection
Typical WBC Count	Often elevated (leukocytosis)	Often normal or low (leukopenia)
Dominant Cell Type	Neutrophils (neutrophilia)	Lymphocytes (lymphocytosis) or can cause neutropenia
Relative Neutrophil Count	High percentage, often >70-90%	Variable, may be low or normal as a percentage
Neutrophil-to-Lymphocyte Ratio (NLR)	High	Often lower than in bacterial infections, but can be high in severe cases

It's important to remember these are general trends. Some severe viral infections can cause high neutrophil counts, and certain bacterial infections might lead to low neutrophil counts if the bone marrow is suppressed.

Why a normal neutrophil count with fever is still possible

Even with a bacterial infection and fever, the total white blood cell count might be normal if other types of white blood cells, such as lymphocytes, decrease while neutrophils increase. This highlights the value of a complete blood count with differential, which details the levels of each white blood cell type, providing a more complete picture of the immune response.

The importance of clinical context

An increase in neutrophils with fever is a frequent and often beneficial immune response, but it should always be considered within the broader clinical context. A single lab result is just one piece of information; healthcare providers use it along with symptoms, vital signs, and patient history to make a diagnosis and treatment plan. A fever with a low neutrophil count (febrile neutropenia) is a medical emergency requiring immediate attention due to the weakened immune defense, and is particularly common after cancer chemotherapy.

Conclusion

In most instances, neutrophils do increase in fever, especially when the cause is a bacterial infection or inflammation. This rise, known as neutrophilia, is a crucial part of the body's defense. Fever can directly enhance neutrophil activity, and inflammatory signals boost their production. However, the specific changes in blood cell counts depend on the underlying cause, and in some cases, fever can occur with low neutrophils. Interpreting these findings within the complete clinical picture is vital for accurate diagnosis and effective care.

For more detailed information on blood cell counts and their health implications, refer to reputable medical resources like the Cleveland Clinic's information on blood disorders.

Frequently Asked Questions

Neutrophilia is a condition characterized by an abnormally high number of neutrophils, the most common type of white blood cell, in the bloodstream.

No, while fever often correlates with high neutrophils, especially in bacterial infections, it's not a rule. Some severe viral infections or conditions like neutropenic fever can cause a fever with a low neutrophil count.

Yes, although typically associated with bacterial infections, some viral infections can cause high neutrophils, especially in the early stages. However, viral infections are more commonly associated with an increase in lymphocytes and sometimes a decrease in neutrophils.

Febrile neutropenia is a medical emergency defined as a fever in a person with a low neutrophil count. It is a serious complication, often resulting from chemotherapy, because the body's immune defenses are severely compromised.

The neutrophil-to-lymphocyte ratio (NLR) is a biomarker that indicates the balance between neutrophils and lymphocytes. A high NLR can be a sign of inflammation or severe infection.

Neutrophilia itself is not treated; it is a sign of an underlying problem. The treatment focuses on addressing the root cause, such as prescribing antibiotics for a bacterial infection or managing inflammation.

A high neutrophil count is a temporary, normal reaction to stress or infection and is usually not dangerous in itself. However, persistently high levels can indicate a chronic condition that requires medical attention.