Polymorphonuclear Leukocytes (PMNs) are a category of white blood cells that represent a rapid-response defense force within the body’s innate immune system. These cells are produced continuously in the bone marrow and circulate throughout the bloodstream, acting as first responders to injury and infection. The defining structural feature of a PMN is its multi-lobed nucleus, which gives rise to the term “polymorphonuclear,” meaning “many-shaped nucleus.” PMNs are constantly on patrol, ready to migrate quickly from the blood into tissues to neutralize foreign invaders like bacteria and fungi. Their immediate and non-specific action is a fundamental part of the body’s initial defense before the adaptive immune system mounts a more specialized response.
What Defines PMN Cells?
PMN cells are also known as granulocytes because their cytoplasm is filled with granules containing powerful enzymes and antimicrobial compounds. These cells are distinguished primarily by the shape of their nucleus and how their granules react to laboratory stains. The three main types of PMN cells are neutrophils, eosinophils, and basophils.
Neutrophils are the most abundant type of white blood cell, typically making up between 40% and 75% of the total white blood cell count. Their nucleus is highly segmented, often appearing to have two to five lobes connected by thin strands. When stained in the laboratory, the granules within a neutrophil’s cytoplasm take on a neutral pink or lilac color.
Eosinophils are less common (1% to 6% of white blood cells), and their nucleus usually has a distinct bi-lobed appearance. Their granules contain proteins that stain brightly with the acidic dye eosin, causing them to appear brick-red or pinkish-orange. Basophils are the rarest (less than 1% of the total), and their nucleus is typically bean-shaped or irregular. Basophil granules stain dark blue or purple with basic dyes, and these granules are so large they often obscure the underlying nucleus.
The Central Function of Neutrophils
As the most numerous PMN cells, neutrophils perform the immediate defense work against acute bacterial infection and inflammation. Their first step is a process called chemotaxis, where the cells sense and follow chemical signals, such as cytokines and bacterial byproducts, that are released at the site of infection. This chemical gradient guides the neutrophils out of the bloodstream and directly toward the invading microorganisms in the tissue.
Once a neutrophil reaches the target, it initiates phagocytosis, engulfing the pathogen. The neutrophil extends portions of its cell membrane (pseudopods) to surround the bacterium, sealing it within a phagosome. The phagosome then fuses with the cell’s granules, flooding the compartment with toxic enzymes and antimicrobial peptides.
The neutrophil then initiates the respiratory burst, rapidly consuming oxygen to generate reactive oxygen species. These highly destructive molecules, such as superoxide and hydrogen peroxide, work to chemically destroy the trapped pathogen inside the phagosome. This intense killing action is highly effective but often results in the rapid death of the neutrophil itself, with the accumulation of dead neutrophils, bacteria, and tissue debris forming pus.
A secondary, highly specialized killing method involves the release of Neutrophil Extracellular Traps (NETs). When facing certain large pathogens or overwhelming infection, the neutrophil can expel its decondensed nuclear material, including its DNA and associated proteins. This sticky, web-like structure traps and immobilizes bacteria and other microbes in the surrounding tissue fluid. While trapping pathogens, the NETs simultaneously release high concentrations of antimicrobial proteins to ensure the destruction of the captured microbes.
When PMN Counts Are Abnormal
PMN cell counts are often measured as part of a Complete Blood Count (CBC) differential, providing physicians with clues about a patient’s health status. An elevated PMN count, particularly an increase in neutrophils, is referred to as neutrophilia. Neutrophilia typically signals an acute process, such as an ongoing bacterial infection, physical trauma, or significant physiological stress.
Bacterial infections trigger the rapid release of stored neutrophils from the bone marrow into the circulation to combat the threat. High counts can also be associated with chronic inflammation, certain medications, or conditions like leukemia.
The presence of low PMN counts, known as neutropenia, is often a serious finding because it indicates a compromised ability to fight infections. Neutropenia can be caused by viral infections, such as influenza, which temporarily suppress bone marrow production, or by exposure to chemotherapy drugs that target rapidly dividing cells. Autoimmune disorders, where the body mistakenly attacks its own neutrophils, or severe, overwhelming infections that deplete the cell reserves can also result in dangerously low counts.
Abnormal counts of the less common PMN types also provide specific diagnostic information. For example, an increase in eosinophils, or eosinophilia, often points toward allergic reactions, asthma, or parasitic worm infections. Basophils, despite their low number, play a role in hypersensitivity reactions, and an increase in their count may suggest certain inflammatory or blood disorders. Interpreting these counts helps providers determine if the body is mounting an appropriate immune response or if an underlying condition is affecting blood cell production or function.