Polymorphonuclear cells (PMNs), also known as granulocytes, are a major component of the body’s innate immune system and act as a rapid, first line of defense against invading pathogens. These specialized white blood cells, or leukocytes, originate in the bone marrow and circulate through the bloodstream. Their primary function is to quickly locate, engulf, and neutralize foreign threats, such as bacteria and fungi, at the site of infection or injury. PMNs are short-lived but highly mobile, making them the body’s dedicated first responders in acute inflammation.
Defining Characteristics and Structure
The name polymorphonuclear literally means “many-shaped nucleus” and describes the cell’s most distinguishing physical feature. Unlike other white blood cells that have a single, rounded nucleus, the PMN nucleus is segmented into two to five distinct lobes connected by thin strands of chromatin. This multi-lobed structure gives the nucleus a highly variable appearance. Due to this unique nuclear morphology, PMNs are sometimes referred to as segmented cells, or “segs,” in clinical settings.
Another defining trait of PMNs is the presence of numerous small sacs, called granules, dispersed throughout the cytoplasm. These granules are storage containers filled with powerful enzymes and antimicrobial proteins. The contents of the granules are released during an immune response to destroy pathogens or regulate inflammation. Because of these prominent cytoplasmic granules, PMNs are also classified collectively as granulocytes.
The Three Granulocyte Types
Polymorphonuclear cells are categorized into three distinct types—neutrophils, eosinophils, and basophils—based on the staining properties and contents of their granules. These three types represent the granulocyte family. They are distinguished by how their granules react to specific dyes used in laboratory staining, which affects their appearance under a microscope.
Neutrophils
Neutrophils are the most abundant type of PMN, typically making up 40% to 70% of all circulating white blood cells. They are the primary cells in the fight against acute bacterial infections and are the first to migrate to a site of injury. Their granules stain a neutral, pale lilac or pink color, which is how they received their name. Neutrophils are highly mobile and are the main component of pus, a collection of dead neutrophils and engulfed microbial debris.
Eosinophils
Eosinophils are far less common than neutrophils, generally comprising 1% to 6% of the white blood cell population. They are primarily involved in defending the body against parasitic infections, such as helminths, by releasing toxic proteins stored in their granules. Eosinophils also play a role in modulating allergic reactions and asthma. Their granules strongly attract the acidic dye eosin, causing them to stain a characteristic bright red or pink color.
Basophils
Basophils are the least numerous of all white blood cells, usually accounting for less than 1% of the total circulating leukocytes. Their primary function is to mediate acute inflammatory and allergic responses. Basophils release powerful mediators, such as histamine and heparin, from their large granules upon activation. These chemicals increase blood flow and vascular permeability to recruit other immune cells to the area. The granules stain intensely with basic dyes, appearing dark blue or purple, often obscuring the bi-lobed or S-shaped nucleus.
Primary Function in Immune Response
The primary role of PMNs is to eliminate invading microorganisms through a regulated sequence of actions. This response begins with chemotaxis, the process by which PMNs are drawn to the site of infection by following a chemical trail. Pathogens and injured host cells release specific signaling molecules, such as cytokines and complement proteins, which create a concentration gradient that guides the PMNs into the affected tissues.
Once the PMN reaches the target, its main mechanism of action is phagocytosis, or “cell eating,” where the cell actively engulfs the foreign particle. The PMN extends its membrane to surround the pathogen, sealing it into a membrane-bound compartment called a phagosome. The phagosome then fuses with the cell’s granules to form a phagolysosome, where the pathogen is subjected to a destructive environment.
The destruction of the invader is achieved through degranulation, the release of the toxic contents of the granules into the phagolysosome. These contents include lytic enzymes and reactive oxygen species, which rapidly break down the engulfed microorganism. PMNs can also release their granule contents externally to kill pathogens too large to be ingested, though this process can cause surrounding tissue damage.
Clinical Significance of Cell Counts
The number of polymorphonuclear cells in the blood is a standard diagnostic tool, typically assessed as part of a routine complete blood count (CBC) with differential. The absolute neutrophil count (ANC) is the most frequently analyzed PMN value, since neutrophils comprise the majority of the granulocyte population. Fluctuations in these counts often provide medical professionals with clues about a patient’s health status and the nature of an infection or inflammatory condition.
An elevated PMN count, particularly neutrophilia, usually indicates an acute bacterial infection or significant inflammation. Conditions such as trauma, burns, or inflammatory diseases can trigger the bone marrow to rapidly release large reserves of PMNs into the circulation. Conversely, a lower-than-normal PMN count, known as neutropenia, suggests a compromised immune capacity.
Neutropenia can result from the immune system being overwhelmed by a severe infection, but it is also a common side effect of chemotherapy or medications that suppress bone marrow production. A low count makes an individual more susceptible to serious infections because the body lacks its first line of defense. Monitoring these cell counts helps guide treatment decisions, such as determining the need for antibiotics or adjusting immunosuppressive therapies.