leukocytes

Leukocytes, or white blood cells, are cells of the immune system that circulate in the bloodstream and reside in lymphoid and other tissues. They originate from hematopoietic stem cells in the bone marrow and are produced under the influence of cytokines and growth factors. Leukocytes are generally categorized as granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes and monocytes). Neutrophils are the most abundant, acting as first responders to infection by phagocytosing microbes. Eosinophils combat parasitic infections and participate in allergic responses. Basophils release mediators such as histamine during inflammatory reactions. Lymphocytes include T cells, B cells, and natural killer cells and mediate adaptive immunity. Monocytes migrate into tissues to become macrophages or dendritic cells, which phagocytose pathogens and present antigens to T cells.

Leukocytes have varying lifespans, from a few hours for some neutrophils to years for memory lymphocytes. They traffic between blood, lymphatic vessels, and tissues, migrating through capillary walls in a process called diapedesis. Normal total leukocyte count in adults ranges roughly from 4,000 to 11,000 cells per microliter, with a differential count indicating the proportion of each type. Abnormal increases or decreases are termed leukocytosis and leukopenia, respectively and can indicate infection, inflammation, or hematologic disorders.

Laboratory tests such as the complete blood count with differential measure leukocyte numbers and help diagnose conditions including leukemia, infections, and immunodeficiencies. Treatments that boost leukocyte production, such as granulocyte colony-stimulating factor, are used in certain clinical contexts.