Medical Definition of Neutrophil. Neutrophil: A type of white blood cell, a granulocyte that is filled with microscopic granules, little sacs containing enzymes that digest microorganisms. Also known as polymorphonuclear leukocyte or poly.
Definition of Neutropenia. Neutropenia: Not enough neutrophils, neutrophils being a type of white blood cell (specifically a form of granulocyte) filled with neutrally-staining granules, tiny sacs of enzymes that help the cell to kill and digest microorganisms it has engulfed by phagocytosis.
Background. Neutropenia is a decrease in circulating neutrophils in the nonmarginal pool, which constitutes 4-5% of total body neutrophil stores. Most of the neutrophils are contained in the bone marrow, either as mitotically active (one third) or postmitotic mature cells (two thirds).