Neutrophil
Neutrophils are a crucial type of white blood cell that play an essential role in the body’s immune system, primarily in fighting infections. They are the most abundant white blood cells, constituting about 60% of the total white blood cell count, and are typically the first responders to sites of infection or injury. Produced in the bone marrow, neutrophils have a short lifespan, lasting only a few hours to a couple of days in tissues. Their primary function involves engulfing and destroying invading microorganisms through a process known as phagocytosis, aided by enzymes stored in their granules.
Neutrophils also release reactive oxygen species to kill nearby pathogens and can form neutrophil extracellular traps (NETs) when they die, which further aid in trapping and destroying microorganisms. The body maintains a delicate balance in neutrophil levels; abnormal increases (neutrophilia) or decreases (neutropenia) can lead to serious health issues. Neutrophilia may occur due to infections or medication reactions, while neutropenia is often seen in cancer patients undergoing chemotherapy or can result from various medical conditions. Understanding neutrophils is vital for comprehending how the immune system functions and responds to threats.
On this Page
Subject Terms
Neutrophil
A neutrophil is a type of white blood cell, an infection-fighting cell that plays a vital role in the body's immune system. Neutrophils are the most numerous white blood cells in the body, and the first to arrive at the site of an infection or injury. They are produced in the bone marrow and are short-lived, with life spans measured in hours. They travel through the body via the bloodstream and function as natural assassins, killing their target microorganisms in a number of ways. Abnormally high or low numbers of neutrophils in the body can result in several blood disorders and can be caused by genetic factors, immune disorders, or a response to medication or infection.
Background
Blood is a multifunctional bodily fluid responsible for transporting oxygen to the body's cells, removing waste products from those cells, and defending against infection and injury. It is made up of four components and moves throughout the body in a system of blood vessels called arteries and veins. One of those components, plasma, is a mostly water-based fluid that carries the blood cells through the body. These blood cells make up the other three components. Red blood cells are disk-shaped cells that carry a protein called hemoglobin. Hemoglobin absorbs and transports oxygen from the lungs and brings it to the other cells in the body. Red blood cells also remove carbon dioxide waste from the body and transport it to the lungs to be exhaled. Platelets are cells that form tiny clots in the blood to stop bleeding.
White blood cells, also called leukocytes, function as infection fighters, defending the body against foreign invaders such as viruses, fungi, bacteria, or parasites. Five types of white blood cells make up about 1 percent of the body's blood. Monocytes are the largest white blood cells and act as scavengers, digesting foreign organisms and even other dead white blood cells. Lymphocytes produce proteins called antibodies that help defend against illness. Eosinophils release toxins that attack and kill parasites and other pathogens. High eosinophil counts are linked to allergic reactions. Basophils are the least numerous type of white blood cells and act as a warning system to alert the body to the presence of invading organisms. They do this by releasing an organic compound called histamine, which widens the blood vessels and allows other white blood cells to reach the site of the infection. Like eosinophils, they are also associated with allergies.
Overview
Neutrophils are the fifth, and most common, type of white blood cells. They make up about 60 percent of the body's count, or about 2,000 to 7,500 cells per cubic millimeter of blood. Like all blood cells, neutrophils are produced in the bone marrow, a soft, springy substance in the center of bones. It takes about a week to form a mature neutrophil. Once produced, neutrophils live for only about four to ten hours in the bloodstream and one to two days if they travel into the body's tissue. Newly formed cells have a round nucleus, but as they age, the nucleus transforms into an irregular lobe shape, which may look like it has splintered into multiple nuclei. Neutrophils, along with basophils and eosinophils, are types of white blood cells called granulocytes, so named because they contain small "granules," or grain-like shapes, within their structures. These granules help neutrophils travel throughout the body and act as storage facilities for the microbe-fighting enzymes within the cell. Enzymes are proteins that produce a chemical reaction in organic substances.
When the body detects an invasion by foreign organisms, it sends out a chemical alarm. The neutrophils are the first to respond, traveling through the bloodstream to the site of the infection. Once there, they attach themselves to the walls of blood vessels and enter the surrounding tissue to attack the invaders and try to stop any more organisms from entering the blood. When they encounter an invading substance, neutrophils can employ an arsenal of antimicrobial weapons to destroy their target.
A primary strategy is a process called phagocytosis, or "cell-devouring." The neutrophil surrounds and engulfs the invading substance and then releases the enzymes from its granules to break down and digest the microorganism. The granules of a neutrophil contain a number of enzymes, some of which weaken the microbial cell wall while others alter its structure or consume the microbe. Neutrophils can release a burst of reactive oxygen to kill a number of surrounding invaders at the same time. This burst emits highly active oxygen-based compounds—such as superoxide or hydrogen peroxide—that destroy the invading cells. Even in dying, neutrophils have developed a method to kill their targets. Under certain conditions a neutrophil can undergo a process called NETosis, a form of cell death that releases a mesh-like network of fibers containing antimicrobial enzymes and chemicals. Scientists have called these neutrophil extracellular traps (NET), though the process that creates them is not fully understood.
Normally, the body regulates production of neutrophils and other white blood cells to respond to infection or injury. Too many or too few neutrophils in the bloodstream can result in sometimes serious blood disorders. Neutrophilia occurs when the number of neutrophils in the blood climbs abnormally high. This condition can result from an infection or a reaction to medication. More common is a disorder called neutropenia, which is an abnormally low number of neutrophils in the blood. Neutropenia is most often found in cancer patients undergoing chemotherapy but can be a congenital genetic condition or a response to medication, immune disorders, or viruses. Neutropenia may be a symptom of other illnesses such as hepatitis, Lyme disease, malaria, and sepsis.
Bibliography
Amulic, Borko, et al. "Neutrophil Function: From Mechanisms to Disease." Annual Review of Immunology, vol. 30, 2012, pp. 459–89, isites.harvard.edu/fs/docs/icb.topic1290469.files/annurev‗immunol%20Zycklinsky.pdf. Accessed 20 Jan. 2017.
Bain, Barbara J. Blood Cells: A Practical Guide. 5th ed., Wiley Blackwell, 2015.
Gabrilovich, Dmitry, editor. The Neutrophils: New Outlook for Old Cells. 3rd ed., Imperial College P, 2013.
"Low White Blood Cell (Neutrophil) Counts and the Risk of Infection." American Cancer Society, www.cancer.org/treatment/treatments-and-side-effects/physical-side-effects/infections/infections-in-people-with-cancer/low-wbc-and-weak-immune-system.html. Accessed 19 Jan. 2017.
Mayadas, Tanya N., Xavier Cullere, and Clifford A. Lowell. "The Multifaceted Functions of Neutrophils." Annual Review of Pathology, vol. 9, Jan. 2014, pp. 181–218, www.ncbi.nlm.nih.gov/pmc/articles/PMC4277181/. Accessed 19 Jan. 2017.
Territo, Mary. "Neutrophilic Leukocytosis." Merck Manual, www.merckmanuals.com/home/blood-disorders/white-blood-cell-disorders/neutrophilic-leukocytosis. Accessed 19 Jan. 2017.
"Types of White Blood Cells." New Health Advisor, www.newhealthadvisor.com/Types-of-White-Blood-Cells.html. Accessed 20 Jan. 2017.
"White Blood Cell Disorders." Dana-Farber/Boston Children's Cancer and Blood Disorders Center, www.danafarberbostonchildrens.org/conditions/blood-disorders/white-blood-cell-disorders.aspx. Accessed 19 Jan. 2017.