Classification and Main Functions
Based on the presence or absence of specific granules in their cytoplasm, all leukocytes are divided into two major groups. The first group comprises granulocytes, which include neutrophils, eosinophils, and basophils. The second group comprises agranulocytes, which lack such granules and include lymphocytes and monocytes.
The primary purpose of these nucleated blood cells is defense. This is carried out in the body through several critical mechanisms:
- Phagocytosis. The active engulfment and destruction of various microorganisms, foreign particles, and the debris of the body's own tissues.
- Immunity. Leukocytes mediate both humoral and cell-mediated immune responses.
- Regulation. These cells are capable of producing and inactivating numerous biologically active substances.
Unique Properties of Leukocytes
A key feature of white blood cells is their pronounced motility. This complex movement is powered by specialized intracellular contractile proteins — actin and myosin.
A vital property of leukocytes is diapedesis — the ability to actively leave the bloodstream. They emigrate from vessels by squeezing directly between endothelial cells. To successfully cross the basement membrane of the endothelial layer, leukocytes secrete specialized enzymes that temporarily degrade its structure.
Directed cell movement within tissues is called chemotaxis or chemokinesis. This migration is strictly regulated by chemical substances known as chemoattractants, which direct cell movement toward a site of inflammation or injury.
Leukocytosis
Any variation in the number of white blood cells is measured per unit volume of circulating blood. An elevated leukocyte count is termed leukocytosis, which is classified into several types depending on the underlying cause:
- Functional (physiological). Occurs in healthy individuals. It is observed during pregnancy, after a meal (postprandial leukocytosis), or following prolonged physical exertion (myogenic leukocytosis).
- Reactive. Develops as an adaptive response to various pathologies. Most commonly accompanies inflammatory processes and massive cell/tissue damage (such as myocardial infarction, stroke, or severe soft-tissue trauma).
- Pathological. Occurs in specific hematologic malignancies, such as leukemias. Its primary and most dangerous feature is a profound disruption of the normal functional activity of the circulating cells.
Leukopenia
Leukopenia is a reduction in the number of leukocytes per unit volume of circulating blood.
By origin, it can be primary (congenital) or secondary. The secondary form arises as a consequence of adverse external factors. The main causes include radiation injury, severe intoxication, and the administration of certain medications.
The clinical significance of this condition requires special attention. Pronounced leukopenia inevitably leads to a sharp decline in patient immunity. This impairs both humoral and cell-mediated immune defense mechanisms and significantly suppresses phagocytosis, leaving the body vulnerable to infections.