White blood cells are largely colorless and transparent when viewed individually in the bloodstream. Unlike red blood cells, they lack the red-pigmented hemoglobin protein responsible for carrying oxygen. These specialized components circulate in the blood and lymph tissue, acting as the primary agents of the immune system. Their main purpose is to locate, identify, and neutralize foreign invaders like bacteria and viruses.
The Reason for the Name
The name “white blood cell” is a historical classification based on how the cells appear when separated from whole blood. When a blood sample is spun in a centrifuge, the elements separate by density. Red blood cells settle at the bottom, and plasma, the liquid portion, floats to the top.
Between these two layers forms a thin, pale, grayish-white layer known as the “buffy coat,” which comprises the white blood cells and platelets. This collective layer of non-red cells gave rise to the common name. The scientific term for these components, leukocytes, reflects this pale appearance, derived from the Greek word leukos, meaning white.
Individually, the unstained cells are translucent under a microscope, lacking any distinct color or major internal pigment. The term “white” therefore refers to the pale layer they form as a group, contrasting sharply with the deep red layer of oxygen-carrying cells below them.
The Body’s Primary Defense Force
White blood cells patrol the body, initiating and managing the immune response against pathogens, damaged cells, and foreign debris. When tissue is injured or infected, specialized cells release chemical signals called chemotaxins. These signals create a chemical gradient that guides the white blood cells to the location of the threat.
This directed movement toward the chemical signal is known as chemotaxis. Once they reach the infection site, cells like neutrophils and monocytes leave the bloodstream by squeezing through the vessel walls in a process called transmigration or diapedesis. This exit from the circulation is necessary for mounting an effective defense, often leading to the swelling and redness characteristic of the inflammatory response.
Phagocytosis is the cellular process of engulfing and destroying targets. Phagocytic cells wrap their membranes around a pathogen, drawing it inside a membrane-bound sac called a phagosome. This sac then fuses with a lysosome, an organelle containing hydrolytic enzymes, forming a phagolysosome. The enzymes and highly reactive molecules, generated in an oxidative burst, break down the engulfed material, neutralizing the threat and cleaning up cellular debris.
Diverse Types and Microscopic Appearance
The white blood cell population is categorized into two main groups: granulocytes and agranulocytes. Granulocytes (neutrophils, eosinophils, and basophils) are named for the prominent chemical-filled granules visible in their cytoplasm. Agranulocytes, consisting of lymphocytes and monocytes, have fewer or no visible granules.
Scientists differentiate these five major types not by their actual color, but by applying specific laboratory stains that bind differently to cellular components. These dyes color the nucleus and the granules, making the cells distinguishable under a light microscope. For example, the nucleus of a neutrophil is distinctly multi-lobed, while the granules stain a pale pinkish-violet.
Eosinophils are identified by their bi-lobed nucleus and large granules that absorb the acidic dye eosin, staining them a bright red or pink color. Basophils, conversely, have large granules that take up basic dyes, resulting in a deep blue or purple appearance that often obscures the nucleus. The agranulocytes are distinguished by their nuclear shape: lymphocytes have a large, nearly spherical nucleus that takes up most of the cell volume, and monocytes possess a distinctive kidney-shaped or C-shaped nucleus.