Neutrophils are a type of white blood cell that forms a part of the innate immune system. They are the most abundant leukocyte in most mammals, acting as the body’s first line of defense against invading pathogens. These cells are among the first to be dispatched to the site of an infection or injury. Their primary role is to fight off infections, particularly those caused by bacteria.
The Role of Neutrophils as First Responders
When tissues are damaged or invaded by pathogens, they release chemical signals that attract neutrophils. This process, known as positive chemotaxis, guides neutrophils to leave the bloodstream and enter the affected tissue. This migration from blood vessels into tissue is a process called diapedesis.
During diapedesis, neutrophils change their shape to squeeze through the small gaps between the cells lining the blood vessels. This allows them to quickly accumulate in large numbers at the site of infection or inflammation. They are the first wave of immune cells to arrive, setting the stage for a broader immune response.
Once they have exited the bloodstream, neutrophils follow the trail of chemical signals, moving toward the highest concentration of these attractants. This targeted migration allows for a swift and localized defense against threats before they can spread further throughout the body.
Methods of Attack Against Pathogens
Upon arriving at the site of an infection, neutrophils employ several methods to combat pathogens. One of their primary functions is phagocytosis, a process where they engulf foreign invaders like bacteria and fungi. The neutrophil’s membrane surrounds the pathogen, enclosing it within an internal compartment called a phagosome. Once trapped inside, the pathogen is destroyed by enzymes and reactive oxygen species.
Another method of attack is degranulation. Neutrophils contain granules filled with antimicrobial proteins and enzymes. When faced with pathogens that are too large to be engulfed, neutrophils can release the contents of these granules directly into the surrounding environment. This release of toxic substances helps to kill the invaders.
A strategy used by neutrophils is the formation of Neutrophil Extracellular Traps (NETs). In this process, a neutrophil can expel its own DNA, which is studded with antimicrobial proteins, to create a web-like structure. This “net” physically traps pathogens, such as bacteria and fungi, preventing their spread and leading to their destruction. This mechanism allows neutrophils to neutralize threats even after the neutrophil itself has died.
The Neutrophil Life Cycle
Neutrophils are produced in the bone marrow, with an estimated 100 billion new cells generated each day. They go through several developmental stages before being released into the bloodstream. Once in circulation, neutrophils have a short lifespan, lasting from a few hours to a few days. Recent studies suggest their lifespan in the blood might be longer, around 5.4 days.
This short lifespan ensures that neutrophils are constantly replenished, providing a ready supply to respond to new threats. After their time in circulation or after engaging with pathogens, aged neutrophils are removed from the body. This clearance is primarily carried out by macrophages in the spleen, liver, and bone marrow.
The removal of old neutrophils is important to prevent the release of their toxic contents, which could damage healthy tissues. This regulated cycle of production, function, and clearance maintains an effective immune defense. The expression of a specific receptor, CXCR4, on the surface of aging neutrophils signals that they are ready to be cleared.
When Neutrophil Levels Are Abnormal
The number of neutrophils in the blood can provide information about a person’s health. An elevated neutrophil count, a condition known as neutrophilia, is a normal response to an infection, particularly a bacterial one. It can also be caused by inflammation, physical stress, or certain medical conditions like rheumatoid arthritis. This increase reflects the body’s effort to fight off a perceived threat.
Conversely, a low neutrophil count, or neutropenia, can leave an individual more vulnerable to infections. Neutropenia can be caused by a variety of factors, including certain viral infections like influenza or hepatitis, autoimmune disorders, and vitamin deficiencies. It can also be a side effect of medical treatments like chemotherapy, which can interfere with the bone marrow’s ability to produce new blood cells.
Abnormal neutrophil levels are a sign of an underlying issue, not a diagnosis in themselves. Both neutrophilia and neutropenia require further investigation by a healthcare professional to determine the cause and appropriate course of action.