polymorphonuclear leukocyte
Meaning of polymorphonuclear leukocyte: Named for a nucleus that looks like several joined beads, these are the first white cells to reach an infection.
Definition of polymorphonuclear leukocyte
A polymorphonuclear leukocyte is a white blood cell with a nucleus divided into several connected lobes and a cytoplasm full of granules. Neutrophils, eosinophils and basophils all fit that description. These cells are the immune system's rapid responders, reaching a site of infection or injury within hours and releasing the enzymes and toxic chemicals stored in their granules.
Where does the name come from?
Under a microscope the nucleus of these cells is not a single round blob. It is pinched into two to five lobes joined by thin threads, so it seems to take a different shape in every cell. Poly means many, morph means shape, and nuclear refers to the nucleus. The lobed shape is not decoration: it lets the cell fold itself through the narrow gaps between the cells lining a blood vessel and reach the tissue beyond.
Which cells are included?
| Cell | How the granules stain | Chief role |
|---|---|---|
| Neutrophil | Faintly, with neither dye | Attacks bacteria and fungi, and is by far the most numerous |
| Eosinophil | Bright pink with acidic dye | Acts against parasites, drives allergic inflammation |
| Basophil | Dark blue with basic dye | Releases histamine, contributes to allergy |
All three are granulocyte cells. The two names describe one group from different angles, one naming the granules and the other the nucleus.
What do they actually do?
A neutrophil is drawn toward trouble by chemical signals released at the site. It sticks to the vessel wall, crawls between the lining cells and follows the signal to its source. There it swallows bacteria into a pouch and empties granule contents in, killing what it has caught. It can also spray enzymes outside itself and fling out a web of its own DNA to trap microbes. That approach damages nearby tissue as well, which is what makes pus and what makes an inflamed area sore. Each cell lives only about a day and dies at the scene, while the marrow replaces it continuously.
Why is the count watched?
A low neutrophil count leaves the body open to bacterial infection, which is why it is followed closely during chemotherapy. A high count usually signals infection or inflammation, though stress and steroid medicines raise it too. Because neutrophils dominate this group, laboratories often use PMN as shorthand for the neutrophil alone, so a fluid sample reported as full of PMNs generally means neutrophils. These cells belong to the wider leukocyte family and form the front line of the immune system.
Also known as
Used in a sentence
Analysis of the joint fluid showed a high polymorphonuclear leukocyte count.
Good to know
PMN is often used loosely to mean neutrophil, since neutrophils are by far the most numerous cell in the group, but the term strictly takes in eosinophils and basophils as well.
Word origin
Where “polymorphonuclear leukocyte” comes from
From the Greek polys, meaning many, and morphe, meaning shape, with leukocyte from leukos, meaning white, and kytos, meaning cell.