Polymorphonuclear Leukocytes (PMNs) are a major population of white blood cells and a fundamental part of the innate immune system. These cells are the rapid-response force, providing the non-specific, immediate line of defense against invading pathogens. PMNs are continuously produced in the bone marrow and circulate through the bloodstream. They are quickly deployed to any site of injury or infection, where they offer immediate protection against foreign invaders.
Defining the Polymorphonuclear Leukocyte Family
The term “polymorphonuclear” describes the cell’s distinct, multi-lobed nucleus, which appears as multiple nuclei under a microscope. PMNs are also known as granulocytes because their cytoplasm contains numerous enzyme-filled granules that stain intensely with specific dyes. The three primary types of PMNs are classified based on how these granules react to laboratory stains. Neutrophils are the most numerous white blood cell, characterized by granules that stain neutrally, typically accounting for 60-70% of total leukocytes. Eosinophils (1-4%) and basophils (less than 1%) make up the remainder of the circulating PMN population.
How Neutrophils Lead the Acute Immune Response
As the most abundant PMN, neutrophils are the primary responders to acute bacterial infection, mobilizing rapidly through a process called chemotaxis. They sense chemical signals released by damaged tissue and invading microbes, allowing them to migrate from the bloodstream into the infected tissue. Once they reach the site of invasion, neutrophils employ a method of destruction known as phagocytosis, which involves the cell engulfing the foreign pathogen into a specialized internal compartment. The microbe is then destroyed by toxic chemicals and enzymes released from the internal granules in a process called degranulation.
Neutrophils also utilize a method of pathogen containment called NETosis, which results in the formation of Neutrophil Extracellular Traps (NETs). These traps are web-like structures made of decondensed chromatin and antimicrobial proteins extruded from the cell. NETs physically ensnare bacteria and fungi, preventing their spread and concentrating them for destruction.
Specialized Roles of Eosinophils and Basophils
Eosinophils and basophils perform specialized immune functions that are distinct from the acute bacterial defense led by neutrophils. Eosinophils are primarily associated with the body’s defense against parasitic infections, such as helminths, which are too large to be cleared by phagocytosis. They release highly toxic proteins from their granules, including Major Basic Protein, which damages the outer surface of the parasite. Eosinophils also contribute to the inflammatory responses seen in allergic diseases like asthma and hay fever.
Basophils are the least common type of PMN but are important in immediate hypersensitivity reactions and inflammation. When activated, typically by an allergen, basophils release pharmacologically active mediators, most notably histamine. Histamine increases blood flow and vascular permeability, which helps recruit other immune cells to the affected area. Basophils also contribute to the immune response against parasites and are a source of immune-regulating molecules.
Clinical Interpretation of PMN Counts
The count of PMNs, especially neutrophils, is a routinely measured component of the Complete Blood Count (CBC) with differential, offering significant diagnostic information. An elevated neutrophil count, a condition known as neutrophilia, is the most common indicator of an acute bacterial infection or physical stress. Conversely, a low neutrophil count, or neutropenia, can signal a compromised immune system, often due to bone marrow issues, certain medications, or viral infections, leaving the body vulnerable to serious infections.
Elevated counts of the less common PMNs provide specific diagnostic clues. Eosinophilia, an increase in eosinophils, frequently points toward an allergic reaction, asthma, or a parasitic infestation. An increase in basophils, known as basophilia, is less frequent but can be seen in certain inflammatory conditions or blood disorders.