A leukocyte is a type of cell that circulates in the blood, where its role is in defending the body from foreign substances. It is commonly called a "white blood cell" to differentiate it from the erythrocytes, or "red blood cells" that carry the distinctly red molecule, hemoglobin. There are numerous kinds of leukocytes, with different roles and different levels of maturity within those roles.
Leukocytes, examined in living tissues, have relatively little pigmentation. While the most modern way of differentiating among types of leukocyte is flow cytometry, for many years, the standard method of determining the relative percentages of leukocyte types was microscopic examination of a thin film of blood. The blood was dried and most often stained with Wright's Stain, which contains red and blue dyes.
Certain leukocytes, such as the eosinophil, are called that because, when stained, they have many intracellular structures that take up the red dye, eosin. In normal individuals, the most common leukocyte is the neutrophil, which is not strongly colored by the stain.
One way to categorize leukocytes is by their microscopic morphology:
Lymphocytes are of multiple subtypes, but some of their major roles are in recognizing foreign substances, and generating chemicals, such as immunoglobulins, which either directly neutralize foreign proteins, or tag hostile substances with opsonins, which signal other cells to physically attack the invaders.
Some leukocytes directly attack foreign particles. Neutrophils and macrophages, the latter being a descendant of monocytes, "eat" the invader, wrapping around it and digesting it in the process of phagocytosis. Eosinophils cause particles too large for phagocytosis to clump together through the process of agglutination, which allows the cell-invader complex to be filtered from the blood.