leukocyte

A leukocyte, or white blood cell, is a cellular component of blood and a central element of the immune system. Leukocytes originate in the bone marrow from hematopoietic stem cells and are released into the bloodstream, where they patrol for signs of infection or tissue damage. In adults, the normal total leukocyte count typically ranges from about 4,000 to 11,000 cells per microliter.

Leukocytes are divided into two main groups: granulocytes and agranulocytes. The granulocytes—neutrophils, eosinophils, and basophils—are distinguished by cytoplasmic granules and play roles in innate immunity and inflammatory responses. Neutrophils are the most abundant and act as primary phagocytes during bacterial infections. Eosinophils participate in responses to parasites and allergies, while basophils release mediators that contribute to inflammation. The agranulocytes—lymphocytes and monocytes—are essential to adaptive and longer-term immune defense. Lymphocytes include B cells, which produce antibodies; T cells, which regulate immune responses and kill infected cells; and natural killer cells, which provide cytotoxic activity. Monocytes move into tissues and differentiate into macrophages and dendritic cells, which help with phagocytosis and antigen presentation.

Leukocytes circulate briefly in the blood, then migrate through capillary walls to sites of infection or injury in a process called diapedesis. They act through phagocytosis, antibody production, antigen presentation, cytokine signaling, and cytotoxic mechanisms. Abnormal leukocyte levels can signal health problems: leukocytosis denotes elevated counts, often from infection or inflammation, while leukopenia indicates reduced counts, which can occur with bone marrow suppression or certain diseases. The leukocyte differential count measures the relative proportions of each type and aids diagnosis.