Neutrophil

A neutrophil is a white blood cell of the granulocyte family, the group whose cytoplasm is packed with small granules. Neutrophils are the most plentiful leukocytes, usually roughly 50 to 70 percent of the white cells in a blood count. Under the microscope they show a nucleus split into several lobes joined by thin strands, which is why they are also called polymorphonuclear leukocytes, and fine pale granules that take up neither acidic nor basic dyes strongly, which gave the cell its name. They are produced in red bone marrow, and each one lives only hours to a few days.

Neutrophils are among the first cells of innate immunity to arrive at an infection or injury. Drawn by chemical signals, they squeeze between the cells of capillary walls into the tissue, engulf microbes by phagocytosis and kill them with enzymes, reactive oxygen compounds and antimicrobial proteins called defensins. They can also cast out neutrophil extracellular traps, nets of DNA studded with toxic proteins that snare microbes. Dead neutrophils, bacteria and debris together make up pus.

A raised neutrophil count, neutrophilia, often points to bacterial infection or inflammation. A low count, neutropenia, which can follow chemotherapy, leaves a person open to serious infections.