What are Polymorphonuclear Leukocytes (PMNs)?

Types of PMNs in Immunity and Allergic Reactions

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

Definition: Polymorphonuclear Leukocytes (PMNs)

Polymorphonuclear leukocytes (PMN), or granulocytes, are a type of white blood cell.   These cells are the bodies first line of defense against infections.

PMNs are referred to as granulocytes because of their appearance under the microscope. When they are stained for examination, the cytoplasm (area between the nucleus and cell membrane) of the cell takes on a grainy appearance.


Types of Polymorphonuclear Leukocytes - PMNs

PMNs include neutrophils, basophils and eosinophils, which circulate in the bloodstream, as well as mast cells (like basophils), which reside in the tissues. Their primary role is to digest any foreign invaders, and create an inflammatory response in response to tissue damage.

Origin of PMN's

PMNs as well as other types of white blood cells, red blood cells, and platelets all descend from hematopoietic stem cells in the bone marrow.  From hematopoietic stem cells, white blood cells differentiate along 2 different lines: the lymphoid cell line which goes on to become lymphocytes, and the myeloid cell line, which goes on the specialize into the different type of PMN's.  The shape of the nucleus in cells in the myeloid line (polymorphonuclear) contrasts with the "mononuclear agranulocytes" of the lymphoid line.

Function of Polymorphonuclear Leukocytes.

The function of PMN's in the blood includes:

The function of the PMN found in tissues:

PMN's and Immunity

The cells classified as PMN's are part of the non-specific innate immune system.  What this means is that they aren't specific - they treat all intruders in the same fashion regardless.  The term innate means that this system is present from birth - the cells don't need to learn to recognize foreign microorganisms as being bad.

This contrasts with the acquired immune system.  In the acquired immune system, the cells can "learn" to recognized an invader by forming antibodies (B lymphocytes,) and in fact that is the mechanisms behind immunizations.

Conditions Involving Abnormal Levels of PMN's

  • An excess of neutrophils is most often caused by infections, as these cells are called on to defend the body.  A deficiency of neutrophils (neutropenia) can be caused by conditions which interfere with the production of neutrophils in the bone marrow, such as chemotherapy-induced neutropenia.
  • 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.

Also Known As: granulocyte, granular leukocyte


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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