Polymorphonuclear cells (PMNs) are a specialized type of white blood cell and a key component of the body’s innate immune system. Characterized by a multi-lobed nucleus and a cytoplasm filled with granules, PMNs are among the first responders to infection or injury. They act as frontline defenders, swiftly identifying and neutralizing foreign invaders.
Key Types of Polymorphonuclear Cells
Polymorphonuclear cells primarily consist of three main types: neutrophils, eosinophils, and basophils, each distinguished by their specific staining properties and specialized functions. Neutrophils are the most abundant PMNs, making up 50% to 70% of all circulating white blood cells. They possess a nucleus divided into two to five lobes and contain granules that stain a neutral, pale lilac color. Neutrophils are the first responders to bacterial and fungal infections, migrating rapidly to sites of inflammation and typically have a short lifespan in circulation.
Eosinophils are another type of PMN, identifiable by their bilobed nucleus and large granules that stain a distinctive reddish-orange with acidic dyes. These cells play a role in the body’s defense against parasitic infections and are involved in allergic reactions.
Basophils are the least common type of PMN, comprising less than 1% of the total white blood cell count. They feature an S-shaped or bilobed nucleus and possess large granules that stain a deep purple with basic dyes. Basophils are involved in allergic responses and inflammatory processes, primarily by releasing substances like histamine and heparin from their granules.
Immune System Roles
Polymorphonuclear cells contribute to the immune system through several mechanisms. One primary function is phagocytosis, a process where these cells engulf and internalize foreign particles like bacteria, fungi, or cellular debris. Once engulfed, these pathogens are contained within a vesicle where they are destroyed by enzymes and reactive oxygen species.
Another mechanism is degranulation, where PMNs release the contents of their cytoplasmic granules into the surrounding environment or directly into phagosomes. These granules contain a diverse array of antimicrobial substances, including enzymes and antimicrobial peptides.
PMNs also play a role in initiating and propagating inflammatory responses. Upon encountering pathogens or tissue damage, these cells are rapidly recruited to the affected site through a process called chemotaxis, guided by chemical signals released by other immune cells or the pathogens themselves. This swift migration allows them to act as the body’s initial cellular defense.
Beyond direct killing, PMNs contribute to inflammation by producing and releasing various cytokines and chemokines. These signaling molecules recruit other immune cells to the site of infection and regulate the immune response. Some neutrophils can also form neutrophil extracellular traps (NETs), which are web-like structures composed of DNA and antimicrobial proteins that ensnare and neutralize pathogens outside the cell.
Impact on Health and Disease
Polymorphonuclear cells are important in maintaining overall health, through their role in fighting infections and contributing to wound healing. Neutrophils, as first responders, rapidly infiltrate wound sites. Here, they remove cellular debris and prevent microbial colonization, ensuring a sterile environment for tissue repair.
Despite their beneficial roles, dysregulation in PMN activity can contribute to various disease states. An excessive number or inappropriate activation of PMNs can lead to chronic inflammation and tissue damage. The release of enzymes and reactive oxygen species, intended to destroy pathogens, can inadvertently harm host tissues if not properly controlled.
PMNs are also implicated in allergic reactions and conditions like asthma. Eosinophils and basophils, in particular, contribute to the inflammatory cascades seen in allergies. They release histamine and other mediators that cause symptoms such as swelling, itching, and airway constriction. In severe asthma, both eosinophils and neutrophils can become activated, releasing inflammatory mediators that damage airway cells.
Dysregulated PMN function can contribute to autoimmune diseases, including rheumatoid arthritis and systemic lupus erythematosus, where the immune system mistakenly attacks the body’s own tissues. Conversely, a deficiency in PMNs can leave an individual susceptible to severe and recurrent infections.