{"id":6534,"date":"2026-06-03T13:22:59","date_gmt":"2026-06-03T17:22:59","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/pathophysiology\/?post_type=chapter&p=6534"},"modified":"2026-06-10T15:19:48","modified_gmt":"2026-06-10T19:19:48","slug":"white-blood-cells-types-and-roles","status":"web-only","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/pathophysiology\/chapter\/white-blood-cells-types-and-roles\/","title":{"raw":"Section 2: White Blood Cells - Types and Roles","rendered":"Section 2: White Blood Cells – Types and Roles"},"content":{"raw":"

Hematopoiesis:\u00a0 Formation of Blood Cells<\/strong><\/span><\/h3>\r\n

Hematopoiesis<\/strong> is the formation of blood cells.\u00a0 Production begins at day 7 of the embryonic life in the yolk sac,<\/strong> migrates to the liver<\/strong> and spleen<\/strong> at week 7, and moves to the bone marrow<\/strong> at ~week 20.\u00a0 The pluripotent stem cells giving rise to all blood cell types are called hemocytoblasts.<\/strong>\u00a0 Their daughter cells differentiate into erythrocytes<\/strong> (RBCs), megakaryocytes<\/strong> (platelet precursors), and leukocytes<\/strong> (WBCs).<\/span><\/p>\r\n\r\n\r\n[caption id=\"attachment_6777\" align=\"alignnone\" width=\"300\"]\"All<\/a> Hematopoiesis:\u00a0 All the formed elements of the blood arise by differentiation of hematopoietic stem cells in the bone marrow.[\/caption]\r\n

Red Blood Cells (Erythrocytes)\u00a0<\/span><\/strong><\/p>\r\n

Erythropoiesis<\/strong> is the production of RBCs.\u00a0 Developing erythroblasts<\/strong> become increasingly packed with hemoglobin.<\/strong>\u00a0 At the normoblast<\/strong> stage, they lose their nucleus, become reticulocytes,<\/strong> enter the bloodstream 2-3 days later and mature into erythrocytes<\/strong> over the following 2 days.\u00a0 Main function:\u00a0 use hemoglobin to transport 98.5% of the oxygen and ~23% of the carbon dioxide in blood.\u00a0 Lifespan ~120 days; aged cells are recycled by macrophages in the liver, bone marrow, and spleen.<\/p>\r\n\r\n[caption id=\"attachment_6524\" align=\"alignnone\" width=\"103\"]\"Erythropoiesis:<\/a> Erythropoiesis: begins with hemocytoblasts producing proerythroblasts that mature into erythroblasts \u2192 normoblasts reticulocytes erythrocytes.
[1] In this diagram, the reticulocyte on the right shows characteristics when stained with methylene blue.
[2] The erythrocyte on the right represents its appearance under a light microscope.[\/caption]\r\n\r\n[caption id=\"attachment_6522\" align=\"alignnone\" width=\"231\"]\"Erythrocyte<\/a> Erythrocyte Life Cycle[\/caption]\r\n

Erythropoiesis is regulated by erythropoietin (EPO)<\/strong> hormone, secreted by the kidneys in response to low blood-oxygen levels, growth hormone (GH), thyroxine, and testosterone.\u00a0 In an adult, 2-3 million RBCs are produced per second and RBC production requires B vitamins, folate, amino acids and iron.<\/p>\r\n\r\n

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Diagnostic Blood Tests<\/strong><\/p>\r\n\r\n<\/header>\r\n

\r\n\r\nBlood Count Terminology<\/span><\/strong>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Retic count<\/strong><\/td>\r\nProportion of RBCs that are reticulocytes (normal: 1-2%).\r\n\r\nLow retic count (e.g., 0.5%) indicates reduced erythropoiesis.<\/td>\r\n<\/tr>\r\n
Hematocrit<\/strong><\/td>\r\nPercentage of blood volume made up of formed elements.\r\n\r\nFemales ~42%;\u00a0Males ~46%\u00a0(testosterone stimulates RBC production).<\/td>\r\n<\/tr>\r\n
Leukopenia<\/strong><\/td>\r\nReduced WBC production.<\/td>\r\n<\/tr>\r\n
Thrombocytopenia<\/strong><\/td>\r\nReduced platelet production.<\/td>\r\n<\/tr>\r\n
Neutrophilia<\/strong><\/td>\r\nElevated neutrophil count - common with bacterial infections.<\/td>\r\n<\/tr>\r\n
Neutropenia<\/strong><\/td>\r\nDecreased neutrophil count - caused by severe infection, nutritional deficiency, congenital defects, autoimmune disease, or cancer.<\/td>\r\n<\/tr>\r\n
Eosinophilia<\/strong><\/td>\r\nElevated eosinophil count - caused by helminth infections, allergies, or autoimmune diseases.<\/td>\r\n<\/tr>\r\n
Lymphocytosis<\/strong><\/td>\r\nElevated NK, T, and P lymphocytes count - occurs with viral infections.<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n<\/div>\r\n\r\n[caption id=\"attachment_6768\" align=\"alignnone\" width=\"300\"]\"Five<\/a> Five types of leukocytes: from left to right, lymphocyte, basophil, eosinophil, neutrophil, and monocytes.[\/caption]\r\n\r\n[caption id=\"attachment_6669\" align=\"alignnone\" width=\"300\"]\"Cells<\/a> Cells of the blood include (1) monocytes, (2) lymphocytes, (3) neutrophils, (4) red blood cells, and (5) platelets. Note the very similar morphologies of the leukocytes (1, 2, 3). (credit: modification of work by Bruce Wetzel, Harry Schaefer, NCI; scale-bar data from Matt Russell).[\/caption]\r\n\r\nPlatelets (Thrombocytes)\u00a0<\/span><\/strong>\r\n\r\nMegakaryocytes<\/strong> fragment into ~1,000 anuclear platelets<\/strong> (thrombocytes).\u00a0 Despite lacking a nucleus, platelet contain ~300 chemicals involved in hemostasis<\/strong> (blood clotting) and are essential for innate defense and preventing excessive bleeding.\r\n\r\nWhite Blood Cell Lineages\u00a0<\/span><\/strong>\r\n\r\nWBCs are classified as either granulocytes<\/strong> (containing large granules, readily visible with light microscope); also called polymorphonuclear leukocytes (PMNs) or agranulocytes<\/strong> (containing smaller granules).\u00a0 Granulocytes include: neutrophils, eosinophils, mast cells<\/strong>, and basophils.<\/strong> \u00a0Agranulocytes include: monocytes, macrophages, dendritic cells<\/strong>, Natural Killer lymphocytes (NK cells<\/strong>), T lymphocytes (T cells<\/strong>), and B lymphocytes (B cells<\/strong>).\u00a0 All WBCs perform amoeboid movement<\/strong> and diapedesis<\/strong> (emigration from blood vessels into tissue - also termed extravasation<\/strong> or transmigration.<\/strong>\r\n

Granulocytes (Polymorphonuclear Leukocytes, PMNs)<\/strong><\/span><\/h3>\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Neutrophils<\/strong><\/span><\/td>\r\nImmature band cells<\/strong> mature into neutrophils.\r\n\r\nMost abundant WBC (50-70%).\u00a0 2-5 lobed nucleus; stain neutral pink with H&E stain.\u00a0 First responders to sites of damage.\r\n\r\nPhagocytose<\/strong> bacteria and utilize extensive lysosomes.<\/strong>\r\n\r\nNeutrophil Oxidative Bursts<\/strong> exocytose highly toxic, unstable Reactive Oxygen Species (ROS)<\/strong> from secretory vesicles that damage bacterial cell walls<\/strong>.\r\n\r\nNeutrophil granules release lactoferrin<\/strong> (sequesters iron, depriving bacteria of required resource)\r\n\r\nNeutrophils release defensin<\/strong> and proteases<\/strong> (degrade pathogen)\r\n\r\nCan also release NETs<\/strong> (Neutrophil Extracellular Traps - webs of chromatin fibers and toxic enzymes) to trap and destroy microbes extracellularly.\r\n\r\nActive lifespan: 24-48 hours.<\/td>\r\n<\/tr>\r\n
Eosinophils<\/strong><\/span><\/td>\r\nStain red with acidic eosin dye;\u00a0 ~2-3% of WBCs.\u00a0 Arrive 2-3 hours after neutrophils.\r\n\r\nPhagocytose<\/strong> debris and pathogens.\r\n\r\nExocytose toxins ROS, Eosinophil Cationic Protein (ECP), <\/strong>and Major Basic Protein (MBP)<\/strong>\u00a0to kill organisms too large to phagocytose (e.g., helminths).\r\n\r\nRelease cytokines<\/strong> to stimulate mast cell and basophil inflammatory response.\r\n\r\nRelease growth factors (e.g., VEGF, vascular endothelial growth factor)<\/strong> to support healing.\r\n\r\nRelease RNases<\/strong> to destroy viruses.\r\n\r\nPlay a role in allergies and asthma<\/td>\r\n<\/tr>\r\n
Mast cells<\/strong><\/span><\/td>\r\nTissue-resident cells, most prevalent in skin dermis, lung mucosa, and GI tract mucosa.\r\n\r\nContain granules with heparin (anticoagulant)<\/strong> and histamine<\/strong>.\r\n\r\nActivated by cellular injury;\u00a0 degranulate releasing pro-inflammatory mediators<\/strong> (histamine, bradykinins, prostaglandins, leukotrienes).<\/strong><\/td>\r\n<\/tr>\r\n
Basophils<\/strong><\/span><\/td>\r\nCirculating counterpart to mast cells<\/strong>.\u00a0 Both contain heparin<\/strong> and histamine granules<\/strong> and fulfill similar pro-inflammatory<\/strong> roles.\u00a0 Basophils circulate the bloodstream rather than residing in tissue.<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n*ROS<\/strong> = Reactive Oxygen Species, such as superoxide, O2<\/sub>\u2022\u2212<\/sup><\/strong> and hydroxyl radical HO\u2022<\/sup><\/strong> are free radicals with unpaired electrons<\/strong> that damage DNA, lipids and proteins.\r\n\r\n \r\n\r\n[caption id=\"attachment_6513\" align=\"alignnone\" width=\"300\"]\"Neutrophil<\/a> Neutrophil Production[\/caption]\r\n\r\n[caption id=\"attachment_2325\" align=\"alignnone\" width=\"150\"]\"Neutrophils<\/a> Neutrophils circulate the bloodstream and are an essential part of the innate (non-specific) immune system.[\/caption]\r\n\r\n[caption id=\"attachment_2324\" align=\"alignnone\" width=\"237\"]\"Neutrophil<\/a> Neutrophil (in purple) migrates from the blood vessel into the matrix using diapedesis. Neutrophils can secrete collagenase (green) in the extracellular matrix of tissues to facilitate tissue remodeling, wound healing and immune cell migration.[\/caption]\r\n\r\n[caption id=\"attachment_6514\" align=\"alignnone\" width=\"300\"]\"The<\/a> The killing mechanisms of neutrophils: phagocytosis, degranulation, and extracellular traps release.[\/caption]\r\n\r\n \r\n\r\n[caption id=\"attachment_5710\" align=\"alignnone\" width=\"1024\"]\"Neutrophil<\/a> Neutrophil Activation: Neutrophils typically circulate the brainstem until chemokines are secreted by monocytes and macrophages and blood vessel walls stimulate blood vessel endothelial cells to express cell adhesion molecule (selectins) that bind to the carbohydrate ligands on neutrophils. Rolling adhesion of neutrophils occurs, followed by firm attachment and diapedesis in which the neutrophil leaves the blood vessel (extravasation) and enters the tissue. Chemokines released by monocytes and macrophages stimulate chemotaxis of neutrophils to assist in the phagocytosis of debris and any invading bacteria. Neutrophils are able to perform 3 different functions to help contain and eliminate the infecting agent: 1) Phagocytosis (followed by antigen presentation), 2) Secretion of ROS, anti-microbial granules and pro-inflammatory cytokines, and 3) NETosis (in which nuclear or mitochondrial DNA is expelled to trap the pathogen).[\/caption]\r\n\r\n[caption id=\"attachment_6779\" align=\"alignnone\" width=\"300\"]\"Three<\/a> Three of the Four Granulocytes (Neutrophils, Eosinophils and Basophils). Granulocytes can be distinguished by the number of lobes in their nuclei and the staining properties of their granules.[\/caption]\r\n\r\n[caption id=\"attachment_6780\" align=\"alignnone\" width=\"300\"]\"Mast<\/a> Mast Cells are granulocytes and function similarly to basophils by inducing and promoting inflammatory responses. This figure shows mast cells in blood. In a blood smear, they are difficult to differentiate from basophils. Unlike basophils that circulate the bloodstream, mast cells migrate from the blood in to patrol various tissues.[\/caption]\r\n

Agranulocytes: Monocytes, Macrophages, and Dendritic Cells<\/strong><\/span><\/h3>\r\n

Monocytes<\/strong> are immature macrophages that mature within 1-3 days into fixed macrophages<\/strong>, free macrophages<\/strong>, or dendritic cells<\/strong>.\u00a0 Despite being immature, monocytes can phagocytose bacteria, secrete cytokines,<\/strong> and act as Antigen Presenting Cells (APCs)<\/strong>.<\/p>\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Fixed macrophages<\/strong><\/span><\/td>\r\nPatrol tissue beds for debris, cancerous cells, and pathogens.\u00a0 Tissue-specific names:\u00a0 microglia<\/strong> (brain), dust cells<\/strong> (lung alveoli), Kupffer\/stellate cells<\/strong> (liver), histiocytes<\/strong> (vertebrae), Langerhans cells<\/strong> (skin).\r\n\r\nSplenic red pulp macrophages<\/strong> and liver macrophages<\/strong> recycle millions of RBCs daily in process called erythrophagocytosis.<\/td>\r\n<\/tr>\r\n
Free macrophages<\/strong><\/span><\/td>\r\nCirculate the bloodstream; enter tissue beds via diapedesis to remove bacteria, cellular debris, and aged neutrophils.<\/td>\r\n<\/tr>\r\n
Dendritic cells<\/strong><\/span><\/td>\r\nReside in tissues exposed to microbes (skin, mucosa).\u00a0 Once activated, migrate to lymph nodes to function as APCs, stimulating T and B lymphocytes.<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nAgranulocytes play important roles in phagocytosing<\/strong> bacteria, as well as secreting cytokines<\/strong> (glycoprotein messengers) to induce inflammation<\/strong> and\u00a0recruit<\/strong> immune cells to the infected or damaged area.\u00a0 Additionally they release growth factors<\/strong> to promote tissue repair.\r\n\r\n[caption id=\"attachment_6783\" align=\"alignnone\" width=\"300\"]\"Monocytes<\/a> Monocytes are large, agranular white blood cells with a nucleus that lacks lobes. When monocytes leave the bloodstream, they differentiate and become macrophages with tissue-specific properties.[\/caption]\r\n

Steps of Phagocytosis<\/strong><\/span><\/h3>\r\n

Phagocytes (neutrophils, monocytes, macrophages,<\/strong> dendritic cells<\/strong>, eosinophils)<\/strong> remove cellular debris and are able to provide non-specific defence by destroying pathogens through seven ordered steps:<\/p>\r\n\r\n