Polymorphonuclear leukocytes, or PMNs, are a category of white blood cells that serve as a primary component of your body’s initial defense system. They are part of the innate immune system, a non-specific defense mechanism that is active from birth. This system allows PMNs to act rapidly against anything the body identifies as foreign, without needing prior exposure to an invader. As some of the first cells to respond to threats, they are dispatched from the bone marrow to confront sites of infection or injury.
Meet the PMNs: Key Defenders of Your Immune System
The term “polymorphonuclear” directly translates to “varied-shape nucleus” and describes the cell’s most telling physical feature. Unlike many other cell types that have a simple, round nucleus, the nucleus of a PMN is segmented into multiple lobes, often appearing as two or three distinct sections connected by thin strands. PMNs are also known as granulocytes because their cytoplasm is filled with granules, which are small sacs containing proteins and enzymes used to fight pathogens.
These cells originate from hematopoietic stem cells within the bone marrow, the body’s primary site for blood cell production. There are three main types of PMNs: neutrophils, eosinophils, and basophils. Neutrophils are by far the most numerous, making up 50-70% of all circulating white blood cells. Eosinophils are involved in combating parasitic infections and play a part in allergic responses. Basophils, the least common type, are also associated with allergic reactions and release substances like histamine.
How PMNs Fight Infections and Aid Healing
When an infection or injury occurs, PMNs are guided to the precise location through a process called chemotaxis. They detect and follow a trail of chemical signals, such as molecules released by bacteria or signals from damaged tissues, which draw them out of the bloodstream and into the affected area.
Once at the scene, the primary function of neutrophils is to eliminate threats through several mechanisms. A main method is phagocytosis, where the neutrophil engulfs microorganisms, cellular debris, or other foreign particles to be destroyed internally. Another tactic is degranulation, the release of antimicrobial proteins and enzymes stored within their cytoplasmic granules. These substances can kill pathogens directly and break down damaged tissue.
In certain situations, neutrophils deploy a unique strategy known as NETosis. During this process, a neutrophil expels a web-like structure composed of its own DNA, histones, and select granular proteins. This “neutrophil extracellular trap,” or NET, effectively ensnares and neutralizes pathogens like bacteria and fungi, preventing them from spreading. PMNs also release signaling molecules that amplify the inflammatory response, recruiting other immune cells and initiating wound healing.
When PMN Counts Are Abnormal: Understanding the Signals
The number of PMNs, particularly neutrophils, circulating in the blood is a standard measurement in a complete blood count (CBC). An abnormally low count of neutrophils is a condition known as neutropenia. This state can leave the body more vulnerable to infections, as there are fewer first-responder cells available to fight off invaders. Common causes of neutropenia include certain viral infections, chemotherapy, autoimmune disorders where the body attacks its own neutrophils, and some congenital conditions.
Conversely, an elevated count of neutrophils is called neutrophilia. This condition is often a direct response to a bacterial infection, as the bone marrow ramps up production to combat the invading pathogens. Neutrophilia can also be triggered by non-infectious inflammation, significant physiological stress, smoking, and the use of certain medications like corticosteroids. In some instances, a persistently high neutrophil count can be an indicator of more serious underlying conditions, including certain types of leukemia.
PMNs: Beyond Acute Infections and Into Chronic Conditions
The activity of PMNs extends beyond their role as emergency responders to acute infections. In chronic inflammatory diseases, such as rheumatoid arthritis and inflammatory bowel disease (IBD), the persistent presence and activation of neutrophils can become problematic. Instead of resolving an infection and dispersing, their continued release of inflammatory substances and enzymes can contribute to ongoing inflammation and cause damage to the body’s own tissues.
Their role in cancer is complex and can be contradictory. Depending on the specific type of cancer and the environment of the tumor, PMNs can exhibit both anti-tumor and pro-tumor behaviors. In some scenarios, they may help suppress tumor growth, while in others, they can release factors that promote tumor development and metastasis.
Furthermore, dysregulation of PMNs is implicated in various autoimmune disorders. In conditions where the immune system mistakenly targets the body’s own cells and tissues, these leukocytes can be wrongly directed to attack healthy organs. Their destructive mechanisms, normally reserved for foreign invaders, are turned inward, contributing to the tissue injury characteristic of autoimmune diseases.