Polymorphonuclear cells (PMNs) are a distinct and highly active category of white blood cells (leukocytes). They are also commonly referred to as granulocytes due to their defining visual characteristic. Their primary purpose is to act as the rapid-response team of the innate immune system, providing a first line of defense against invading pathogens. PMNs are produced in the bone marrow and circulate in the bloodstream, ready to mobilize quickly to sites of injury or infection.
Shared Structural Characteristics
The name polymorphonuclear literally means “many-shaped nucleus,” which is the most distinguishing feature of this cell group. The nucleus of a PMN is not a simple, round structure, but instead appears lobed, segmented, or multi-shaped. This irregular nuclear morphology helps differentiate PMNs from mononuclear cells like lymphocytes and monocytes.
The second defining trait is the presence of numerous specialized granules packed within the cell’s cytoplasm. These granules contain defensive molecules, including enzymes and chemical mediators. The contents are released during an immune response to neutralize or degrade foreign invaders, linking the cell’s granular appearance directly to its function.
Specific Functions of the Three Granulocytes
Neutrophils
Neutrophils are the most abundant type of PMN, typically making up the largest percentage of circulating white blood cells. They are the principal fighters of acute bacterial infection and are the first immune cells to arrive at an inflamed or infected site. Neutrophils employ phagocytosis, where they engulf and consume bacteria, fungi, and cellular debris.
Once a pathogen is internalized, the neutrophil releases the contents of its granules into the internal vesicle, destroying the microbe with substances like myeloperoxidase. Neutrophils have a short lifespan of only a few days before undergoing programmed cell death. Their high concentration and quick mobilization are decisive factors in controlling the initial spread of infection.
Eosinophils
Eosinophils are less common than neutrophils but play specialized roles in defense against larger parasites and in allergic reactions. These cells contain granules loaded with specific toxic proteins, notably major basic protein, which damages the outer layer of parasitic worms too large to be engulfed. Their presence is associated with conditions like asthma, hay fever, and parasitic infestations.
When activated during an allergic response, eosinophils release mediators, contributing to the inflammation seen in hypersensitivity reactions. Eosinophils are primarily found in the tissues lining the digestive, respiratory, and urinary tracts, which are common entry points for foreign organisms.
Basophils
Basophils are the least numerous of the PMNs, constituting less than one percent of the total white blood cell count. Despite their low numbers, they carry inflammatory mediators, including histamine and heparin. Histamine is a vasodilator that increases blood flow and vessel permeability, helping other immune components reach the site of action quickly.
Heparin acts as an anticoagulant, preventing localized clotting that could impede the movement of immune cells into the affected tissue. Basophils are considered the blood-circulating counterparts of mast cells and are centrally involved in immediate hypersensitivity reactions. Their degranulation process contributes significantly to the symptoms observed in allergic responses.
Laboratory Identification and Differential Counts
The visual differentiation of PMNs is achieved through specialized staining techniques applied to a peripheral blood smear. The standard clinical procedure utilizes the Wright-Giemsa stain, a compound dye containing both acidic and basic components. This stain binds differently to the chemical contents within the granules of each PMN type, revealing distinct color characteristics under a microscope.
Neutrophil granules take up both the acidic and basic dyes, resulting in a neutral, pale pink or lilac coloration, while their nucleus stains dark blue. Eosinophil granules are strongly “acidophilic,” binding intensely to the acidic dye, appearing bright red or orange. Basophil granules are “basophilic,” binding to the basic dye, which results in a distinct dark blue or purple color that often obscures the nucleus.
A differential count is performed to quantify the percentage of each type of white blood cell present in the blood sample. This count provides a snapshot of the immune system’s composition. Deviations from expected proportions are often the first laboratory indication of an underlying health issue, guiding further diagnostic investigation.
Diagnostic Significance of PMN Levels
Changes in the number of circulating PMNs offer important insights into a patient’s health status, as measured by the differential blood count. Neutrophilia, an elevated neutrophil count, is most frequently observed in cases of acute bacterial infection, as the bone marrow rapidly releases reserves. High neutrophil levels can also indicate physical stress, inflammation, or tissue damage.
Eosinophilia, an increase in eosinophils, suggests the body is responding to a parasitic infestation or an allergic disorder such as asthma or drug hypersensitivity. Persistent eosinophilia can sometimes be a sign of certain hematologic conditions. Conversely, basophilia (an elevated basophil count) is a less common finding but may signal chronic inflammation or certain myeloproliferative disorders.
Abnormally low counts, collectively referred to as “penias,” also carry clinical weight. Neutropenia, a low neutrophil count, is concerning because it leaves the patient vulnerable to life-threatening bacterial and fungal infections. Low PMN levels can result from impaired production in the bone marrow, such as from chemotherapy or certain autoimmune diseases, signaling a compromised immune capacity.