Leucocyte

Leucocytes, or white blood cells, are nucleated cells that form a key component of the vertebrate immune system. They circulate in the blood and move into tissues as needed to defend the body against infections, foreign substances, and abnormal cells. Leucocytes originate from hematopoietic stem cells in the bone marrow and can have lifespans ranging from hours to years, depending on the cell type and physiological conditions.

Leucocytes are commonly classified into two broad groups: granulocytes and agranulocytes. Granulocytes include neutrophils, eosinophils, and basophils, which contain granules that release enzymes and mediators during immune responses. Agranulocytes consist of lymphocytes—the principal cells of adaptive immunity, including B cells, T cells, and natural killer cells—and monocytes, which differentiate into macrophages and dendritic cells in tissues. Each type has specialized roles in recognizing, responding to, and removing pathogens or abnormal cells.

In function, neutrophils are rapid responders that perform phagocytosis and microbial killing. Monocytes and macrophages provide sustained defense and antigen presentation. Lymphocytes mediate targeted immune responses: B cells produce antibodies; CD4+ and CD8+ T cells coordinate cellular immunity and cytotoxicity; natural killer cells destroy certain infected or tumorigenic cells. Eosinophils combat parasites and modulate allergic reactions, while basophils release histamine and other mediators that influence inflammation.

Clinical relevance often centers on leucocyte counts and distribution, which can indicate infection, inflammation, or hematologic disorders such as leukemia. Differential counts and functional assays aid in diagnosis and monitoring of immune health.