A leukocyte, or white blood cell (WBC), is a cellular component of the blood that lacks hemoglobin, has a nucleus, is capable of motility, and defends the body against infection and disease. White blood cells are key components of the body's immune system. They are produced in the bone marrow through a process called hematopoiesis and circulate in the blood and lymphatic system, ready to respond to injury or illness. Unlike erythrocyte (red blood cells), leukocytes can leave the circulatory system to migrate to sites of infection or tissue damage.

Etymology

The term leukocyte derives from the Greek words leukos, meaning 'white', and kytos, meaning 'hollow vessel' or 'cell'. The name reflects their appearance after centrifugation of a blood sample, where they form a thin, whitish layer called the buffy coat, situated between the red blood cells and the blood plasma.

Types of Leukocytes

Leukocytes are classified into two main groups based on the presence or absence of visible granules in their cytoplasm: granulocytes and agranulocytes. Leukocyte | Definition, Types, & Function

Granulocytes

Granulocytes possess prominent cytoplasmic granules and have lobed nuclei. They are part of the innate immune system and are typically the first responders to infection. All are produced via the myeloid stem cell line in the bone marrow. White Blood Cells (WBCs)

• –Neutrophil: The most abundant type of leukocyte, comprising 50-70% of the total count. Neutrophils are highly motile phagocytic cells that engulf and destroy pathogens, primarily bacteria. They are one of the first cell types to arrive at a site of acute inflammation.
• –Eosinophil: Making up about 1-4% of leukocytes, eosinophils are involved in combating multicellular parasitic infections. They also play a role in modulating allergic inflammatory responses.
• –Basophil: The least common type, accounting for less than 1% of all leukocytes. Basophils release histamine and other mediators of inflammation, and are associated with allergic reactions and hypersensitivity.

Agranulocytes

Agranulocytes lack visible cytoplasmic granules and have non-lobular, often rounded, nuclei. They are primarily involved in the adaptive immune system.

• –Lymphocyte: The second most common type of WBC (20-40%), lymphocytes are crucial for adaptive immunity. They are further divided into:
  • –B cell: Responsible for humoral immunity. When activated by an antigen, they differentiate into plasma cells that produce antibody molecules.
  • –T cell: Central to cell-mediated immunity. They include helper T cells, which coordinate the immune response, and cytotoxic T cells, which kill infected or cancerous cells. T cells mature in the thymus.
  • –Natural killer cell (NK cell): A type of cytotoxic lymphocyte critical to the innate immune system. NK cells can recognize and kill stressed cells, such as tumor cells and virus-infected cells, without prior sensitization.
• –Monocyte: The largest type of leukocyte (2-8%). Monocytes circulate in the bloodstream for a few days before migrating into tissues, where they differentiate into macrophages or dendritic cells. These cells are powerful phagocytes and act as antigen-presenting cells to activate T lymphocytes. Blood Cells and Their Precursors

Life Cycle and Production

Leukocytes originate from hematopoietic stem cells in the bone marrow. This process, known as leukopoiesis, involves the differentiation of these stem cells into either common myeloid progenitors (which give rise to granulocytes and monocytes) or common lymphoid progenitors (which give rise to lymphocytes). The lifespan of leukocytes varies dramatically; neutrophils may live for only a few hours to days in circulation, whereas some memory lymphocytes can persist for many years. Leukocyte | Definition,Types, & Function

Clinical Significance

The number of leukocytes in the blood is an important diagnostic indicator. A standard blood test, the complete blood count (CBC), measures the total number of white blood cells.

• –Leukocytosis: An elevated white blood cell count, often indicating an active infection, inflammation, severe physical stress, or certain types of cancer, such as leukemia.
• –Leukopenia: A decreased white blood cell count, which makes the body more susceptible to infections. Causes can include viral infections (like HIV), autoimmune disorders, certain medications (like chemotherapy), and bone marrow failure.

The WBC differential is a part of the CBC that reports the percentage of each type of leukocyte. Shifts in these percentages can provide clinicians with more specific information about the underlying medical condition. White Blood Cell (WBC) Count