leukosyytien

Leukocytes, commonly referred to as white blood cells, are essential cells of the immune system responsible for defending the body against infections, foreign substances, and malignant cells. They circulate in the bloodstream and are also found in lymphoid tissues such as the spleen, lymph nodes, and thymus.

Leukocyte production, or leukopoiesis, occurs primarily in the bone marrow from multipotent hematopoietic stem cells. The differentiation of these cells is directed by cytokines and growth factors, culminating in distinct classes of mature leukocytes that are released into the peripheral blood under physiological conditions.

The principal classes of leukocytes are neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Neutrophils constitute the largest fraction and are key players in the early innate immune response, phagocytosing bacteria and releasing granules that contain antimicrobial substances. Lymphocytes, comprising T cells, B cells, and natural killer cells, are central to adaptive immunity, mediating targeted responses and immune memory. Monocytes differentiate into macrophages and dendritic cells within tissues, contributing to phagocytosis, antigen presentation, and cytokine production. Eosinophils and basophils are involved in allergic reactions and defense against parasitic infections.

Leukocytes employ a variety of mechanisms—phagocytosis, degranulation, antibody production, and release of reactive oxygen species—to eliminate pathogens. They also communicate with each other via cytokines and chemokines, coordinating a comprehensive immune response.

Clinically, leukocyte counts are a routine diagnostic tool. Leukocytosis may indicate infection, inflammation, or hematologic disorders, while leukopenia can signal immunosuppression or bone marrow failure. Differential leukocyte counts aid in diagnosing specific conditions: a high neutrophil count suggests bacterial infection, whereas elevated lymphocytes may indicate viral disease or certain leukemias. Monitoring leukocyte dynamics is therefore integral to the assessment, monitoring, and management of a wide range of medical conditions.