WBCs

White blood cells (WBCs), or leukocytes, are immune cells in the blood and lymphatic system that defend the body against infections and foreign substances. They originate from hematopoietic stem cells in the bone marrow and circulate in the bloodstream and tissues, where they mediate immune responses, inflammation, and tissue repair. Their activities include phagocytosis, antibody production, antigen presentation, and regulation of inflammatory processes.

There are five major categories: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Neutrophils are the most abundant and act as early responders to bacterial infection, performing phagocytosis and releasing antimicrobial substances. Lymphocytes comprise B cells, T cells, and natural killer cells, providing adaptive and innate immune functions. Monocytes mature into macrophages and dendritic cells that engulf pathogens and present antigens to other immune cells. Eosinophils combat parasites and participate in allergic reactions, while basophils release histamine and other mediators during inflammation.

Production and lifespan: WBCs are produced in the bone marrow. Their numbers rise in response to infection or inflammation. They circulate in blood for hours to days before migrating into tissues. Lifespans vary by type; neutrophils often survive only a few hours in tissues, whereas some lymphocytes can persist for years.

Clinical relevance: WBC count is a standard part of the complete blood count (CBC). Normal adult ranges are roughly 4,000 to 11,000 cells per microliter. Leukocytosis indicates possible infection, inflammation, or hematologic disorders; leukopenia can reflect bone marrow suppression or immune compromise. A differential count helps diagnose specific conditions by showing the relative proportions of the five types.

Summary: WBCs are essential for host defense and orchestrate immune responses, with diverse roles across innate and adaptive immunity.