Polymorphonuclear Leukocytes White Blood Cells

Polymorphonuclear leukocytes (PMNs) are a type of white blood cell (WBC) that include neutrophils, eosinophils, basophils, and mast cells. PMNs are a subtype of leukocytes, which protect the body against infectious organisms.

PMNs are also known as granulocytes. They play a central role in the innate immune system.

In normal conditions, the most common PMN, by far, is the neutrophil. These make up the most significant amount of blood cells produced by the bone marrow and are the first line of defense in protecting the body from infection.

This article explains PMN's origin, function, and abnormalities.

What Does Polymorphonuclear Mean?

The term "polymorphonuclear" describes the varied shapes and sizes of the cells' nucleus (the part of the cell that contains chromosomes).

PMNs' nuclei have two or three lobes with deep divisions. This dynamic is unlike many other cells where the nucleus has more of a unified "egg yolk" appearance.

PMNs are also called granulocytes or granular leukocytes because they contain and release granules. The contents of granules vary by cell type. In the case of neutrophils, the granules contain proteins and substances that help fight infection.

Histamine is released with mast cells and basophils when the cell degranulates (breaks down). This breakdown triggers a defensive inflammatory response.

Origin of PMNs

PMNs, other types of white blood cells, red blood cells, and platelets develop from hematopoietic stem cells in the bone marrow.

Hematopoietic stem cell precursors are those cells committed to forming a new kind of cell. From precursors, the blood-forming cells follow two pathways:

• Lymphoid cell line, where cells can differentiate to become lymphocytes
• Myeloid cell line, where cells can become different types of PMNs and other blood cells

Except for mast cells (found in connective tissue), PMNs are found primarily in the blood. However, the cells will often follow the immune system's chemical signals and move to different sites in the body where they are needed.

For example, when the body experiences inflammation, blood vessels widen so these cells can more easily reach the site of an infection or injury. PMNs are the body's front-line defense against disease and infection.

Innate vs. Acquired Immune Response

PMNs are part of the non-specific innate immune system. That means that they similarly treat all intruders. 

The term "innate" means that this system can function from birth. The cells don't need to learn to recognize the invaders; they attack anything that the body considers foreign.

The innate immune response differs from the acquired immune response.

Specialized immune cells learn to recognize specific invaders in the acquired immune system. The response is more complex than that of the innate immune response.

The acquired immune response involves:

• B cell lymphocytes, which destroy invading germs
• T cell lymphocytes, which destroy compromised cells in the body
• Antigen-presenting cells (APCs), which alert lymphocytes to foreign agents

Function

Each PMN has a slightly different role in health, although there is some overlap. For example, while a healthy PMN response can fight infection, an inappropriate response (such as releasing histamine in people with allergic asthma) can cause problems.

Neutrophils

Neutrophils are the body's first-line defense against bacteria, viruses, and fungal infections. When there is a tissue injury, the body releases chemotactic factors (substances that stimulate cell migration) to attract neutrophils.

Eosinophils

Eosinophils are involved in allergic reactions and also fight parasitic infections. High levels of eosinophils can result from other conditions, such as drug reactions or immune system disorders like eosinophilic esophagitis.

Basophils

Basophils are also involved in allergic reactions. In addition, they secrete histamine and other compounds that cause inflammation. Basophils are the bloodborne equivalent of mast cells.

Mast Cells

Mast cells live in tissues and play an important role in respiratory and digestive conditions. Mast cells have two major subtypes:

• Connective tissue mast cells, which trigger inflammation
• Mucosal mast cells, which keep the gut in balance

Histamine and other substances within these PMNs (such as heparin) help regulate the immune response.

Abnormal Levels of PMNs

In a complete blood count (CBC), labs use a reference value (RV) for each cell type. Anything below the RV may be considered low, while anything above the reference value may be high.

Some conditions cause low or high levels of PMNs in the blood.

Neutrophilia

High levels of neutrophils in the blood, referred to as neutrophilia, are most often caused by infections. In addition, certain blood cancers result in increased neutrophils. Blood cancers include:

• Chronic myelocytic leukemia
• Polycythemia vera
• Primary myelofibrosis (angiogenic myeloid metaplasia)

Neutropenia

Neutropenia occurs when the body does not have enough neutrophils. When this happens, it can increase a person’s risk of infection. In addition, certain cancer therapies can cause neutrophil levels to fall, resulting in chemotherapy-induced neutropenia.

Eosinophilia

Eosinophilia is the excessive production of eosinophils (a type of white blood cell). It can be the result of:

• Allergic reactions
• Drug reactions
• Parasitic infections

A deficiency of these cells is uncommon.

Basophilia

Basophilia is an excess of basophils (a white blood cell). It may occur with hypothyroidism, blood cancers, and inflammatory bowel disease (IBD) like Crohn's disease and ulcerative colitis. Low basophil counts are also uncommon.

Summary

PMNs are a type of white blood cell. They are part of the innate immune system and attack foreign substances. Blood work can detect unusual levels of PMNs, which may indicate infection or other conditions.