What Are Neutrophils?

Neutrophils are the most common type of white blood cell. Like all white blood cells, they work to fight off infectious organisms in your body, such as viruses and bacteria. They are also called polymorphonuclear (PMN) leukocytes.

Neutrophils can be assessed with a blood test. High or low levels can help diagnose some medical conditions.

This article will discuss what neutrophils do, where they come from, and what high or low neutrophil counts mean.

What Do Neutrophils Do? 

Neutrophils are the first type of white blood cell triggered to help fight against infection. Once they arrive at the location of a pathogen (infectious organism) in the body, neutrophils have several functions that help the body eliminate the pathogen. 

Actions include:

• Signaling other types of white blood cells to aid in combating the infection 
• Destroying the organism by releasing chemicals 
• Helping raise the temperature, which makes it difficult for an infectious pathogen to survive 
• Phagocytosis, which is a process of engulfing pieces of the infectious pathogen 
• Sending out neutrophil extracellular traps (NETs), net-like structures made of nuclear material that entrap pathogens

The broken-down infectious material is removed from the body, and the neutrophils are also broken down.

Where Do Neutrophils Come From?

Neutrophils and all blood cells develop in the bone marrow. Immature neutrophils mature in the bone marrow for about 14 days and then circulate in the blood. In the bone marrow, they appear as juvenile neutrophils with a round nucleus. They also develop granules in their cytoplasm.

As they mature, the nucleus transforms from round to band-shaped, and they are called band neutrophils. A fully mature neutrophil has a segmented nucleus and is called a segmented neutrophil.

Both band and segmented neutrophils may be seen circulating in the blood. A small number of mature neutrophils also reside in some other tissues and organs throughout the body.

Neutrophils have a life span in the bloodstream of approximately one day. They migrate into tissues where needed and still have a short remaining lifespan. The body is constantly making new neutrophils, which replace the older neutrophils.

What Is a Normal Neutrophil Count? 

Neutrophils are typically measured in a complete blood count (CBC) and differential. A CBC is a blood test often done for routine screening or to examine the cause of common symptoms, such as fever, fatigue, swelling, vomiting, difficulty breathing, and more.

There are standard values for the typical number of neutrophils in a blood sample, the percent of white blood cells in a sample that is neutrophils, and how many band or juvenile neutrophils are seen.

In addition to a CBC, sometimes neutrophils are also evaluated with a bone marrow biopsy. This is an invasive test in which a sample of bone marrow is removed. The sample is sent to a lab to examine the number of neutrophils being formed in the bone marrow and any abnormalities in them.

A bone marrow biopsy is not a routine test. It is usually done to evaluate potential blood cancer or other conditions affecting blood cell production or the bone marrow.

What Does It Mean When Neutrophils Are High? 

An elevated number of neutrophils is most commonly a sign of a recent or ongoing infection. Typically, in these instances, neutrophils will be elevated for a short period of time as the body fights an infection, and then the neutrophils will eventually decrease to a normal level.

A higher than usual percentage of band neutrophils compared to segmented neutrophils in a blood count may be seen with an acute infection or acute inflammation as the bone marrow responds by releasing neutrophils earlier in their maturation process.

Having a high neutrophil count due to a resolving infection is not a cause for concern. It is a sign that the body’s immune system is effectively working to protect the body from infectious organisms.

High Neutrophil Count and Disease

In addition to infection, there are other causes of elevated neutrophils. They can increase as a side effect of some medications. Certain medical conditions, such as cancer, allergic reactions, autoimmune disorders, trauma, and heart attack, can cause an increase in the number of neutrophils.

In some of these situations, such as during a heart attack or after an injury, the neutrophils work to help reduce damage and to facilitate healing.

Autoimmune conditions may cause neutrophils or other white blood cells to be elevated as the body attacks its own tissue. This may contribute to the symptoms of autoimmune conditions, such as swelling, fever, and pain. But changes in the neutrophil count with autoimmune diseases are usually not significant enough or consistent enough to be used in diagnosis or disease monitoring. 

What Does It Mean When Neutrophils Are Low? 

Neutropenia is a low neutrophil count. Medical conditions can affect the body’s production of white blood cells or cause them to break down prematurely. Sometimes serious infections can cause neutropenia. Bone marrow disease may affect the production of any type of blood cell, including neutrophils.

Chemotherapeutic medications, which are used to treat cancer, can inhibit blood cell production in the bone marrow and sometimes cause premature destruction of neutrophils and other blood cells, leading to neutropenia.

With a low neutrophil count, the body might not be able to fight infections as it should. This can predispose a person to severe and prolonged infections from viruses, bacteria, fungi, or parasites.

Febrile (high fever) neutropenia is a condition that occurs with some infections, and it can be a sign of a severe bacterial infection that requires medical management.

Conditions that may cause neutropenia include:

• Medications, such as chemotherapy
• Bone marrow damage
• Cancer, especially blood cell cancer
• Congenital (present at birth) disorders of the bone marrow 
• Infections 
• Autoimmune diseases 

Infections, cancer, and autoimmune diseases may increase or decrease neutrophils at different stages of the disease.

Summary 

Neutrophils are the most common type of white blood cell in the blood. They tend to be the first to arrive at a location where the body is fighting a pathogen, such as a virus, bacteria, fungus, or parasite, and they have many different, coordinated ways of protecting the body from infections.

Neutrophils will typically rise during an infection and decrease to normal levels after the infection has resolved. A low neutrophil count is a common side effect of chemotherapy, and this can weaken your immune system.

The concentration of neutrophils in the blood can be measured with a common blood test. In certain situations, a bone marrow biopsy may be necessary to determine whether there is an abnormality with neutrophils as they are developing. 

Changes in your neutrophil count can be associated with a fever, swelling, fatigue, and susceptibility to infections.