Polymorphonuclear Leukocytes (PMNs), also known as granulocytes, are a type of white blood cell central to the body’s innate immune system. The name “polymorphonuclear” refers to the multi-lobed shape of the cell’s nucleus, which distinguishes them from other blood cells. Produced in the bone marrow, PMNs act as the immune system’s rapid-response team, mobilizing first to the site of injury or infection. Their count is routinely measured in a complete blood count test, offering immediate clues about a person’s health status.
The Role of PMNs in Immunity
PMNs include neutrophils, eosinophils, and basophils, though neutrophils account for the vast majority of the circulating count. These cells are professional phagocytes, meaning their primary action is phagocytosis—a process where the cell physically engulfs and consumes foreign substances like bacteria or fungi.
Once a microbe is ingested, the PMN uses specialized granules within its cytoplasm to release enzymes and destructive chemicals that break down the pathogen. PMNs are constantly released from the bone marrow and patrol the body via the bloodstream. At the site of infection or tissue damage, chemical signals called chemotactic substances guide the PMNs out of the blood and directly to the affected area.
This rapid mobilization and destruction makes PMNs the first line of defense in the immune response. Because neutrophils have a short lifespan, often less than 24 hours in circulation, the body must constantly produce and regulate these cells to maintain a functional defense system.
Conditions Leading to Elevated PMN Counts
An elevated PMN count, primarily due to an increase in neutrophils, is medically termed neutrophilia. The most common cause for this rise is an acute bacterial infection, such as pneumonia or an abscess. When bacteria enter the body, the immune system triggers increased production of neutrophils from the bone marrow to fight the invading organisms.
Neutrophilia can also result from significant inflammation or trauma. Conditions like severe burns, crush injuries, or surgical procedures initiate an inflammatory cascade, signaling the need for a large influx of PMNs for cleanup and defense. Autoimmune flare-ups, such as those seen in rheumatoid arthritis or vasculitis, similarly provoke an inflammatory state that elevates the PMN count.
Certain medications directly influence PMN levels by stimulating bone marrow production or causing cells to “demarginate” from blood vessel walls into circulation. Corticosteroids, like prednisone, are a common example, shifting PMNs from the marginal pool back into the central bloodstream. Physical or emotional stress, including vigorous exercise or an anxiety attack, can also cause a temporary, or transient, increase in PMN count due to the release of hormones like epinephrine.
Finally, certain malignancies can cause a sustained elevation in PMNs. This can happen when a tumor outgrows its blood supply, leading to necrosis and inflammation, or when the cancer cells themselves produce growth factors that stimulate PMN production in the bone marrow. Chronic Myeloid Leukemia (CML) and other myeloproliferative disorders are examples of conditions where the bone marrow’s regulation of PMN production is impaired, leading to excessive numbers.
Conditions Leading to Low PMN Counts
A reduction in PMN count, or neutropenia, is a potentially serious condition that increases the risk of infection. Neutropenia is defined as an Absolute Neutrophil Count (ANC) below 1,500 cells per microliter of blood. This decrease results from three main mechanisms: reduced production in the bone marrow, increased destruction of circulating cells, or abnormal sequestration.
Bone marrow suppression is a frequent cause of neutropenia, often resulting from treatments like chemotherapy or radiation therapy, which inhibit new blood cell production. Certain non-cancer medications, including some antibiotics and antithyroid drugs, can also suppress bone marrow activity or trigger an immune response that destroys PMNs. Nutritional deficiencies, such as a lack of vitamin B12 or folate, also impair the bone marrow’s ability to produce healthy neutrophils.
Infections can cause neutropenia, particularly viral infections like influenza, mononucleosis, or hepatitis. These viruses can temporarily suppress the bone marrow or lead to the immune-mediated destruction of circulating PMNs. Conversely, a severe bacterial infection (sepsis) can initially cause neutrophilia, but if the demand exceeds the bone marrow’s supply, it leads to rapid depletion and subsequent neutropenia.
Autoimmune disorders, like Lupus or Rheumatoid Arthritis, can cause the body to mistakenly produce antibodies that target and destroy circulating PMNs. This increased destruction rate leads to lower counts, even if the bone marrow is producing cells normally. Depending on the severity of the neutropenia—moderate (below 1,000 cells/µL) or severe (below 500 cells/µL)—a person’s vulnerability to bacterial and fungal infections rises significantly.
Interpreting PMN Results
PMN counts are typically reported as part of a complete blood count (CBC) with a differential, which breaks down the percentages of all white blood cell types. The most clinically relevant measurement derived from this data is the Absolute Neutrophil Count (ANC), which calculates the total number of neutrophils available to fight infection. The ANC is calculated by multiplying the total white blood cell count by the percentage of PMNs and any immature neutrophils, known as “bands.”
A high percentage of bands, referred to as a “left shift,” suggests the bone marrow is rapidly releasing immature cells in response to an overwhelming or acute infection. While a high or low count indicates an underlying process, the result must always be considered within the context of a person’s symptoms and overall clinical picture. A single lab value cannot provide a diagnosis; only a qualified medical professional can accurately interpret the ANC alongside other blood markers and patient history.