Structure and Function of Leukocytes

A healthy individual has approximately 4,500 to 11,000 leukocytes, or white blood cells, per microliter of blood. This represents about 1% of total blood volume. Unlike red blood cells, leukocytes contain a nucleus and cellular organelles but lack hemoglobin, making them structurally distinct components of the characteristics and functions of blood.

Most leukocytes reside in connective tissues, particularly in lymphatic organs such as the spleen and lymph nodes. Only a small fraction of leukocytes circulate in the bloodstream at any given time. This distribution allows the immune system to maintain protective barriers throughout the body while keeping reserves in strategic lymphoid tissues.

When infection or injury occurs, circulating leukocytes become activated and their numbers can double within a few hours, a process called leukocytosis. Damaged or infected tissues and circulating pathogens release chemical stimuli that attract these activated leukocytes to the affected site, enabling rapid immune response.

Phagocytosis is the process by which certain leukocytes engulf and destroy pathogens and debris. These cells adhere to endothelial cells lining blood vessels and squeeze through to enter tissues. Once in the tissue, they clear infected cells, cellular debris, and circulating pathogens, providing direct immune defense.

Leukocytes protect the body from pathogens, toxins, and abnormal cells as part of the immune system. They accomplish this through phagocytosis, chemical signaling, and direct tissue infiltration. Their ability to respond rapidly to threats and clear infected material makes them essential for maintaining health and preventing disease.

Leukocytes adhere to and interact with endothelial cells lining blood vessels, then squeeze through the vessel wall to enter surrounding tissues. This process, called diapedesis, allows them to reach sites of infection or injury where they can eliminate pathogens and damaged cells directly.

During infection or injury, leukocyte numbers increase through leukocytosis, a rapid mobilization response where circulating white blood cells double within hours. Chemical signals from damaged tissues and pathogens activate bone marrow to release additional leukocytes, amplifying the immune response to combat threats effectively.