White blood cells, or leukocytes, can be classified into five major categories based on morphological and functional characteristics. They may also be classified as granular or agranular based on the presence or lack of granules (small particles) within the cell cytoplasm. Leukocytes defend against invading microorganisms either by stimulating specific cellular or humoral (antibody production) immune responses, or by phagocytosis.
Their names reflect the staining characteristics of the granules present in their cytoplasm. The name polymorphonuclear leukocyte also refers to the number of lobes comprising the nucleus of that cell type.
Monocytes, the fourth group of leukocytes, have few granules and a typically kidney-shaped nucleus. In tissue, monocytes become macrophages. Macrophages are capable of surviving months to years thereby providing important immune surveillance within the tissue of the various organ systems.
The last major group of leukocytes includes the lymphocytes. They are agranular round cells, with a proportionally large nucleus. Lymphocytes are primarily responsible for adaptive or acquired immunity.