A leukocyte, commonly called a white blood cell, is a cell of the immune system that participates in defense against infection and other immune responses. Leukocytes comprise several distinct cell types rather than a single uniform population. They originate from blood-forming precursors in bone marrow and occur in blood, lymphoid tissues, and other tissues throughout the body. Some directly engulf microorganisms, while others recognize particular targets, produce antibodies, or coordinate the activity of neighboring cells. (medlineplus.gov)
Structure and classification
Unlike mature human erythrocytes, leukocytes retain a nucleus. Their size, nuclear shape, and cytoplasmic staining help distinguish them in blood smears. Routine laboratory classification recognizes five principal groups: neutrophils, eosinophils, basophils, monocytes, and lymphocytes. These categories describe the major circulating populations, not every specialized immune cell found in tissues. (histology.leeds.ac.uk)
The traditional morphological division separates granulocytes—neutrophils, eosinophils, and basophils—from agranulocytes, comprising monocytes and lymphocytes. This distinction concerns the prominence of granules under conventional light-microscope staining; it does not mean that agranulocytes lack all intracellular granules. Granulocytes typically have lobed nuclei, whereas lymphocytes commonly have rounded nuclei and monocytes often have indented or kidney-shaped nuclei. (histology.leeds.ac.uk)
Formation and maturation
Leukocyte production forms part of hematopoiesis, the generation of blood cells from hematopoietic stem cells. In the conventional developmental scheme, myeloid lineages produce granulocytes and monocytes, while lymphoid lineages produce lymphocytes. These lineage categories describe developmental relationships and should not be confused with the granulocyte–agranulocyte distinction, which is based on appearance. (ncbi.nlm.nih.gov)
Different populations mature in different locations. Human B lymphocytes develop and mature in bone marrow, whereas T-cell precursors migrate to the thymus for maturation. Mature cells subsequently populate peripheral tissues and lymphoid organs. Bone marrow therefore supplies the circulating immune-cell population, but blood is only one compartment in which leukocytes operate. (histology.leeds.ac.uk)
Major types and functions
Neutrophils are normally the most abundant circulating leukocytes in adults. They are important early participants in responses to infection and tissue injury. Through phagocytosis, they engulf bacteria and other material, using antimicrobial substances to help destroy ingested organisms. Their activity is closely associated with acute inflammation. (histology.leeds.ac.uk)
Eosinophils contain prominent granules that stain reddish with conventional dyes. They participate in responses to parasitic worms and are involved in allergic inflammation. Basophils, usually the least numerous circulating leukocytes, contain dark-staining granules and release mediators such as histamine during immune reactions. Despite functional similarities, basophils and tissue mast cells are distinct cell types. (histology.leeds.ac.uk)
Monocytes are relatively large circulating cells with abundant cytoplasm. After entering tissues, they can differentiate into macrophages, which engulf microorganisms and cellular debris. Macrophages also participate in immune signaling and antigen presentation. The monocyte–macrophage relationship illustrates how a circulating leukocyte can acquire specialized functions in a tissue environment. (histology.leeds.ac.uk)
Lymphocytes include B cells, T cells, and natural killer cells. Activated B cells can differentiate into plasma cells, which secrete antibodies. T cells include populations that coordinate immune responses and populations that kill infected or otherwise abnormal cells. Natural killer cells also have cytotoxic functions but belong to innate rather than conventional antigen-specific adaptive immunity. (ncbi.nlm.nih.gov)
Immune coordination and movement
Leukocytes contribute to both innate immunity and adaptive immunity. Innate responses provide rapid recognition and containment of threats; adaptive responses involve antigen-specific B and T lymphocytes. These systems interact rather than operate independently. For example, dendritic cells capture material in tissues and present antigens to lymphocytes, linking local detection to the initiation of adaptive responses. (ncbi.nlm.nih.gov)
Immune responses require cells to reach the appropriate locations. Leukocytes can leave blood vessels and enter surrounding tissues, while lymphocytes circulate through blood and the lymphatic system. Migration allows immune surveillance and recruitment to sites of infection or injury. Many leukocytes also communicate through cytokines, signaling molecules that influence activation and interactions among immune cells. (ncbi.nlm.nih.gov)
Laboratory measurement and clinical significance
A white blood cell count measures the total concentration of circulating leukocytes and commonly forms part of a complete blood count. A differential count measures the individual leukocyte categories. One commonly cited adult reference interval is 4,500–11,000 cells per microliter, equivalent to 4.5–11.0 × 10⁹ cells per liter, although laboratory reference intervals vary. (medlineplus.gov)
An elevated count is called leukocytosis, and a reduced count is called leukopenia. Changes may accompany infections, inflammatory conditions, marrow disorders, or medication effects. Some blood cancers, including leukemia, affect leukocyte production or development. A total count alone does not establish a diagnosis; interpretation incorporates the differential, other laboratory findings, and clinical circumstances. (medlineplus.gov)
A blood smear complements numerical measurements by showing cell size, shape, and other morphological features. Flow cytometry provides a different kind of information by distinguishing immune-cell populations through their molecular markers, allowing analysis of subsets that may look similar under a microscope. (medlineplus.gov)