Leukocytes (white blood cells)
Leucocytus
Also known as: white blood cell, WBC, white cells, leucocyte, leukocytes, granulocytes, agranulocytes
The nucleated cells of the immune system that travel in the blood: granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (monocytes, lymphocytes), normally 4.0–11.0 × 10⁹/L in adults.
Overview
Leukocytes are named for the pale "buffy coat" they form between plasma and red cells in centrifuged blood (< 1% of blood volume). Unlike erythrocytes they have nuclei and organelles, and most of their working life is spent outside the blood: the circulation is only the highway between the marrow, lymphoid organs and tissues.
They are classified by the appearance of their cytoplasm on a Romanowsky-stained film. Granulocytes have prominent specific granules and lobed nuclei (hence "polymorphonuclear" leukocytes): neutrophils (pale lilac granules), eosinophils (large orange-red granules) and basophils (large dark blue-purple granules). Agranulocytes (mononuclear leukocytes) lack visible specific granules, although they contain small lysosomal (azurophilic) granules: monocytes and lymphocytes. By lineage, granulocytes, monocytes (and mast cells and most dendritic cells) are myeloid, while lymphocytes (T, B, NK) are lymphoid.
To leave the blood, leukocytes follow the leukocyte adhesion cascade: margination towards the vessel wall; rolling on E- and P-selectins expressed by activated endothelium (binding sialyl-Lewis X); activation by chemokines displayed on the endothelium; firm adhesion as integrins (LFA-1, Mac-1, VLA-4) bind ICAM-1 and VCAM-1; transmigration (diapedesis) between endothelial cells, involving PECAM-1 (CD31); and chemotaxis through the tissues towards C5a, leukotriene B4, CXCL8 (IL-8) and bacterial formyl peptides.
- Location
Blood (transit), bone marrow (production and storage), lymphoid organs and every tissue of the body.
Parts & subdivisions
- ≈ 40–75% (2.0–7.5 × 10⁹/L): first-responding phagocytes of acute bacterial infection.
- ≈ 20–45% (1.0–4.0 × 10⁹/L): T, B and NK cells of adaptive (and innate) immunity.
- ≈ 2–10% (0.2–1.0 × 10⁹/L): precursors of tissue macrophages and some dendritic cells.
- ≈ 1–6% (0.04–0.4 × 10⁹/L): helminth defence and allergy.
- < 1% (0.01–0.1 × 10⁹/L): histamine-rich cells of allergic inflammation.
Anatomy
- Histology
Peripheral blood film (Wright/Giemsa or May–Grünwald–Giemsa): neutrophil 3–5-lobed nucleus, fine lilac granules; eosinophil bilobed nucleus, orange granules; basophil obscured nucleus, dark granules; monocyte large, kidney-shaped nucleus, grey-blue cytoplasm; lymphocyte small round dense nucleus with a thin rim of blue cytoplasm.
Function
- Defend against infection by phagocytosis, cytotoxicity and antibody production.
- Initiate, amplify and resolve inflammation through cytokines and lipid mediators.
- Remove dead cells and debris and contribute to tissue repair.
- Provide immune surveillance against malignancy.
- Physiology
Counts vary with age (higher in neonates and children, who have relatively more lymphocytes), pregnancy (neutrophilia), exercise, stress and corticosteroids (neutrophilia from demargination and marrow release; lymphopenia and eosinopenia) and ethnicity (benign ethnic neutropenia associated with the Duffy-null phenotype).
Clinical significance
Leukocytosis and leukopenia
Leukocytosis (> 11 × 10⁹/L) is usually a neutrophilia from bacterial infection, inflammation, tissue necrosis, steroids or stress; very high counts (> 50 × 10⁹/L) suggest leukaemia or a leukaemoid reaction. Neutropenia < 1.5 × 10⁹/L, severe < 0.5 × 10⁹/L: fever in a neutropenic patient (e.g. after chemotherapy) is neutropenic sepsis, a medical emergency requiring immediate broad-spectrum antibiotics.
Leukocyte adhesion deficiency
LAD type 1 (defective CD18, the β2-integrin subunit of LFA-1 and Mac-1) prevents firm adhesion and extravasation: recurrent bacterial infections without pus, very high blood neutrophil counts, poor wound healing and delayed separation of the umbilical cord (> 3 weeks).
Mnemonics
Abundance — "Never Let Monkeys Eat Bananas"
Neutrophils > Lymphocytes > Monocytes > Eosinophils > Basophils (≈ 60 : 30 : 6 : 3 : 1 %).
Granulocytes — "BEN"
Basophils, Eosinophils, Neutrophils are the granulocytes; monocytes and lymphocytes are agranulocytes.
Compiled from standard anatomy and physiology references — see the Lymphatic & Immune System sources. For education only; not medical advice.