What are Polymorphonuclear Leukocytes (PMNs)?

PMNs: A Family of White Blood Cells

What are polymorphonuclear leukocytes (PMNs) and what is their function?. Istockphoto.com/Stock Photo©toeytoey2530

Polymorphonuclear leukocytes, or PMNs, are a special family of white blood cells. The family includes immune cells known as neutrophils, eosinophils and basophils. When it comes your blood stream, however, the most common PMN by far is the neutrophil.

What Does Polymorphonuclear Mean?

PMNs are known for their appearance under the microscope. The nucleus can have deep partial divisions so that it takes the shape of two or three lobes.

These lobed nuclei mean that the “fried egg” appearance, so common with many other cell types, is noticeably absent. That is, the “yolk” of the egg looks like it broke while being cooked.

Are PMNs Granulocytes?

Yes. In addition to funny nuclei, PMNs also have granules that can be seen using dyes and stains; the granules are like tiny water balloons within the cell, and they give PMNs their grainy appearance. For this reason, PMNs are also frequently called granular leukocytes or granulocytes. What’s inside the granules depends on which PMNs you are talking about. In the case of neutrophils, the granules contain enzymes and substances with antimicrobial properties that, when released, help fight infection.

Types of Polymorphonuclear Leukocytes - PMNs

Neutrophils, basophils and eosinophils are PMNs that can be found circulating in the bloodstream. Since white blood cells are known to follow chemical signals, migrating to cites where they are needed, you can also find them in other areas.

All of these PMNs can have slightly different roles in health and in disease. A primary role for granulocytes in health is that of immunity. However, the same immune cells that may be helpful in fighting foreign invaders, may in other instances be implicated in potentially harmful or imbalanced responses, such as allergies and serious allergic responses.

Origin of PMNs

PMNs as well as other types of white blood cells, red blood cells, and platelets all descend or develop from hematopoietic stem cells in the bone marrow.  From hematopoietic stem cells, blood cells differentiate along two major, distinct pathways: the lymphoid cell line which goes on to become lymphocytes, and the myeloid cell line, which gives rise to different types of PMNs, in addition to other blood cells. 

Function of PMNs

The function of PMNs in the blood includes:

  • Neutrophils - Neutrophils are the front line of defense against bacteria, viruses, and fungal infections, and are the first to appear on the scene. When there is a tissue injury, substances called chemotactic factors are released which attract neutrophils.
  • Eosinophils - Eosinophils work in allergic reactions and also against parasitic infections such as worm infestations. High levels of eosinophils in the blood can result from other conditions, too, such as drug reactions.
  • Basophils - Basophils are also involved in allergic reactions, and secrete histamine and other compounds, which produce inflammation and prevent blood from clotting. Basophils are the bloodborne equivalent mast cells, which are strictly tissue-bound.

    Mast Cells in Tissues:

    • Mast cells - In the tissues, mast cells play an important role in respiratory and digestive conditions. Mast cells exist in 2 major subtypes—connective tissue and mucosal—both of which can release preformed granules. Mucosal areas are linings, such as the lining of the intestines, or the lining of the airways. Connective tissue exists deeper than these linings and in other structures, as well. In both subtypes, mast cells may become activated and degranulate, or give up their payload of granules. Substances within their granules act as cell signals and work in concert with other immune cells.

    PMNs and Immunity

    The cells classified as PMNs are part of the non-specific innate immune system. What this means is that they they treat all intruders in similar fashion. The term innate means that this system is present from birth — the cells don't need to learn to recognize the invaders. They recognize them from the get-go.

    This contrasts with the acquired immune system.  In the acquired immune system, the immune cells can learn to recognize an invader and mount an immune response. The response is often complex and multifaceted, involving immune cells known as B and T lymphocytes, as well as antigen presenting cells, or APCs, which specialize in alerting the lymphocytes as to the presence of foreign antigen.

    Conditions Involving Abnormal Levels of PMNs

    • High levels of neutrophils in the blood are most often caused by infections, as these cells are called on to defend the body. 
    • When the body does not have enough neutrophils, this can correlate with a person’s risk of developing an infection. Certain cancer therapies can cause neutrophil levels to fall, resulting in what's known as neutropenia. In chemotherapy-induced neutropenia, health care providers guard against the risk of infection in a variety of ways.
    • An excess of eosinophils is termed eosinophilia, and is often caused by allergic reactions, drug reactions, or infections with parasites, and less commonly by cancer and other conditions.  A deficiency in these cells is uncommon.
    • An excess of basophils may occur with hypothyroidism and with some blood cancers. Other disorders linked to excess basophil levels include Crohn disease and ulcerative colitis—both of which are considered inflammatory bowel disease.

    Sources:

    Kurashima Y, Kiyono H. New era for mucosal mast cells: their roles in inflammation, allergic immune responses and adjuvant development. Experimental & Molecular Medicine (2014) 46, e83.

    Canadian Cancer Society. Anatomy and physiology of the blood. Accessed 02/24/16. http://www.cancer.ca/en/cancer-information/cancer-type/leukemia/anatomy-and-physiology/?region=on

    Williams, L. "Comprehensive Review of Hematopoiesis and Immunology: Implications for Hematopoietic Stem Cell Transplant Recipients" in Ezzone, S. (2004) Hematopoietic Stem Cell Transplantation: A Manual for Nursing Practice Oncology Nursing Society: Pittsburgh, PA (pp.1- 12)

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