Polymorphonuclear Leukocytes White Blood Cells

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

Also known as granulocytes, PMNs play a central role in the innate immune system. In normal conditions, the most common PMN, by far, is the neutrophil.

What Does Polymorphonuclear Mean?

The term polymorphonuclear describes the varied shapes and sizes of the nucleus of these cells.

The nuclei of PMNs have two or three lobes with deep divisions. This is unlike many other cells in which 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 enzymes and substances with antimicrobial properties that help fight infection.

With mast cells and basophils, histamine, a chemical, is released when the cell degranulates (the granules break down), triggering a defensive inflammatory response.

Origin of PMNs

PMNs as well as other types of white blood cells, red blood cells, and platelets all descend (develop from) hematopoietic stem cells in the bone marrow. From hematopoietic stem cell precursors, the blood cell lines differentiate along two distinct pathways:

• Lymphoid cell line: Cells can differentiate to become lymphocytes.
• Myeloid cell line: Cells can become different types of PMNs, in addition to other blood cells.

With the exception of mast cells, PMNs are found primarily in the blood. However, the cells will often follow chemical signals emitted by the immune system and migrate to different sites in the body where they are needed.

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

Function

PMNs are part of the non-specific innate immune system. This means that they treat all intruders in a similar fashion. The term innate means that this system can function from birth. The cells don't need to learn to recognize the invaders; they simply attack anything that the body considers foreign.

Each different PMN has a slightly different role in health, although there is some overlap. While a healthy PMN response can fight infection, an inappropriate response (such as the release of histamine in people with allergic asthma) can have adverse consequences.

The functions of neutrophils and other PMNs are as follows:

• Neutrophils: These cells are the body's first-line defense against bacteria, viruses, and fungal infections. When there is a tissue injury, substances called chemotactic factors are released to attract neutrophils.
• Eosinophils: These cells are involved in allergic reactions and also against parasitic infections. High levels of eosinophils can result from other conditions, such as drug reactions or immune system disorders like eosinophilic esophagitis.
• Basophils: These cells are also involved in allergic reactions and secrete histamine and other compounds that lead to inflammation. Basophils are the bloodborne equivalent of mast cells.
• Mast cells: These cells are found in tissues and play an important role in respiratory and digestive conditions. Mast cells exist in two major subtypes: connective tissue mast cells, which induce inflammation, and mucosal mast cells, which maintain the homeostasis of the gut. Histamine and other substances within these granulocytes (such as the anticoagulant heparin) help regulate the balance of the immune response.

The innate immune response differs from the acquired immune response.

In the acquired immune system, specialized immune cells learn to recognize specific invaders. The response is complex and multifaceted than that of the innate immune response, and it involves B cell and T cell lymphocytes as well as antigen-presenting cells (APCs)—which alert lymphocytes to the presence of a specific foreign agent.

Abnormal Levels of PMNs

Blood tests are measured with a complete blood count (CBC). A reference value (RV) is used for each cell type. Anything below the RV may be considered to be low, while anything above the reference value may be considered high.

There are conditions that cause low or high levels of PMNs in the blood.

These are referred to as:

• Neutrophilia: High levels of neutrophils in the blood, referred to as neutropenia, are most often caused by infections. Certain blood cancers, including chronic myelocytic leukemia, polycythemia vera, primary myelofibrosis (angiogenic myeloid metaplasia) result in increased neutrophils, but there are many noncancerous causes as well.
• Neutropenia: When the body does not have enough neutrophils, referred to as neutropenia, this can increase a person’s risk of infection. Certain cancer therapies can cause neutrophil levels to fall, resulting in chemotherapy-induced neutropenia.
• Eosinophilia: The excessive production of eosinophils, referred to as eosinophilia, can be the result of allergic reactions, drug reactions, or parasitic infections. A deficiency of these cells is uncommon.
• Basophilia: An excess of basophils, referred to as basophilia, may occur with hypothyroidism and with some blood cancers. Other disorders linked to basophilia include Crohn's disease and ulcerative colitis—each of which is classified as an inflammatory bowel disease (IBD). Low basophil counts are also uncommon.