An adult in good health typically has between 4,500 and 11,000 leukocytes, or white blood cells, per microliter of blood, which constitutes about 1% of the total blood volume. Unlike red blood cells, white blood cells contain a nucleus and other cellular organelles but do not have hemoglobin. Most white blood cells reside in connective tissues, particularly in lymphatic organs such as the lymph nodes, with only a small fraction present in circulating blood.

White blood cells protect the body from potential threats such as toxins, pathogens, and abnormal cells. When an infection or injury occurs, the circulating white blood cells become activated, and their numbers can increase rapidly within a few hours, a response known as leukocytosis. Chemical stimuli released by damaged or infected tissues and circulating pathogens attract these activated white blood cells to the site of infection or injury.

Some types of white blood cells are capable of phagocytosis. They can adhere to and interact with endothelial cells lining the blood vessels, squeezing through to enter the tissue. This capability allows them to clear infected tissue cells, debris, and circulating pathogens from the body.