Polymorphonuclear (PMN) cells are a category of white blood cells that are a part of the body’s innate immune system. The name “polymorphonuclear” describes the varied shapes of their nucleus, which is divided into multiple lobes. PMNs are also called granulocytes because they contain granules, small particles filled with proteins that help the body fight infections. They are released from the bone marrow as a first line of defense against infection and inflammation.
The Role of PMN Cells in the Immune System
As the immune system’s first responders, PMN cells are among the initial cells to arrive at a site of injury or infection. Neutrophils, the most common type of PMN, are particularly swift, often reaching the location of an issue within minutes. Their multi-lobed nucleus aids their movement through tight gaps between other cells, allowing them to migrate from the blood into tissues more easily. This rapid deployment is a feature of the innate immune response.
A primary function of PMN cells is phagocytosis, the process of engulfing and destroying foreign invaders like bacteria and fungi. When a PMN cell encounters a pathogen, it extends its cell membrane to surround the microbe, enclosing it within an internal compartment called a phagosome. Once trapped, the PMN releases the contents of its granules into the phagosome. These granules contain destructive enzymes and antimicrobial proteins that break down and neutralize the threat.
Beyond directly attacking pathogens, PMN cells help orchestrate the inflammatory response. When they arrive at a site of infection, they release chemical signals, such as cytokines and chemokines, into the surrounding tissue. These signals act as a call for reinforcements, attracting other immune cells to the area to help combat the infection.
This inflammatory response, while protective, can sometimes cause damage to surrounding healthy tissues. The enzymes and reactive oxygen species that PMNs use to kill microbes can also harm the body’s own cells if released excessively. The timely removal of PMNs after they have performed their function is an important part of resolving inflammation and preventing tissue damage. This cleanup process is handled by other immune cells, such as macrophages.
The Lifecycle of a PMN Cell
All PMN cells originate in the bone marrow from hematopoietic stem cells. This production process, known as hematopoiesis, is a continuous function. The formation of granulocytes is called granulopoiesis. The body produces an enormous number of these cells daily, with estimates around 100 billion neutrophils produced each day under normal conditions.
The development from a stem cell into a mature PMN cell takes approximately 12 to 17 days. This maturation process occurs within the bone marrow, where the cells go through several stages before they are ready to be released. Once mature, a large reserve of PMNs is stored in the bone marrow, which can be quickly mobilized in response to infection or inflammation.
Once released from the bone marrow into the bloodstream, PMN cells have a short lifespan, ranging from a few hours to just over a day. They are not designed to be long-lived; their purpose is to perform a specific function and then be cleared. This rapid turnover ensures the supply of PMNs is constantly refreshed. After a short time in circulation, they are removed by other cells, primarily in the bone marrow, liver, and spleen.
PMN Cell Levels and Health
The number of PMN cells in the blood is measured as part of a complete blood count (CBC). Since neutrophils are the most abundant type, a PMN count is often a reflection of the neutrophil count. Normal levels can vary, but neutrophils make up about 50% to 70% of all white blood cells. Doctors evaluate these levels to gain insight into a person’s immune system.
An elevated level of PMN cells, particularly neutrophils, is a condition known as neutrophilia. This often indicates that the body is fighting a bacterial infection. In response to infection, the bone marrow releases its stored reserves of neutrophils into the bloodstream. Other causes of high PMN levels include inflammation from injury or chronic diseases, stress, and certain medications.
Conversely, a low level of PMN cells, or neutropenia, means the body has fewer neutrophils than normal. This condition makes a person more vulnerable to infections. Neutropenia can be caused by factors including viral infections, autoimmune diseases where the body attacks its own neutrophils, and some medications like chemotherapy. Problems with the bone marrow itself can also lead to insufficient production of these cells.
Types of PMN Cells
While the term “PMN cell” is often used interchangeably with “neutrophil,” it is a category that includes three distinct types of cells. They differ in their specific roles and the types of threats they are specialized to handle. They can be distinguished in the lab by the way their granules appear with specific stains.
Neutrophils are by far the most abundant type of PMN, constituting up to 70% of all circulating white blood cells. They are the primary defenders against bacterial and fungal infections and are the main cells found in pus.
Eosinophils and basophils are much less common in the bloodstream. Eosinophils are involved in fighting parasitic infections and play a role in allergic reactions, such as asthma. Basophils are the least numerous type of PMN and are also associated with allergic responses. They release substances like histamine, which contributes to the inflammatory response seen in allergies.