White Blood Cells

Introduction

• White blood cells arise from distinct stem cell lineages under cytokine control, mature in marrow, circulate briefly, and act in tissues as key defenders—phagocytes, allergy mediators, and immune cells—while their dysregulation leads to age-related patterns of leukemia.

• Leucocytes, or white blood cells, perform essential defence functions by protecting the body against infections and foreign agents.
• Leucocytes are classified into two main developmental groups: myeloid series and lymphoid series.
• Each series arises from distinct hematopoietic precursor cells and follows a separate pathway of maturation.
• The myeloid series differentiates into:
  • Granulocytes, which include neutrophils, eosinophils, and basophils.
  • Monocytes, which later transform into macrophages in tissues.
• The lymphoid series differentiates into lymphocytes, which are key cells of the immune system.
• Although monocytes and lymphocytes may contain cytoplasmic granules, these granules are not clearly visible with routine stains. Therefore, they are classified as agranulocytes.
• Cells of the granulocytic lineage develop from myeloblasts, which initially lack visible granules. During maturation, these cells synthesize proteins that are stored in cytoplasmic granules.
• The granules contain enzymes and antimicrobial substances that contribute to the destruction of pathogens.
• Granulocytes are capable of active movement and migrate to sites of inflammation by amoeboid motility, where they eliminate microorganisms mainly by phagocytosis.
• Neutrophils are highly motile and strongly phagocytic cells that destroy a wide range of microorganisms, particularly bacteria. They act as the first line of defence in acute bacterial infections.
• Monocytes are also phagocytic and play a significant role in prolonged infections. They function as a second line of defence, especially in chronic and parasitic conditions.
• Eosinophils and basophils are specialized cells that participate in allergic and inflammatory reactions.
• Lymphocytes are primarily involved in specific immune responses, including antibody production and cell-mediated immunity.

Life of leucocytes:

• The life cycle of leucocytes can be divided into three phases: marrow phase, circulatory phase, and tissue phase.
• In the marrow phase, leucocytes develop from hematopoietic progenitor cells in the bone marrow.
• During this phase, they undergo proliferation, differentiation, and maturation before being released into the bloodstream.
• The average duration of this phase is approximately 6 to 10 days.
• In the circulatory phase, mature leucocytes circulate in the peripheral blood.
• This phase is relatively short, lasting about 6 to 8 hours for most cells.
• During this period, leucocytes remain available for rapid response to injury or infection.
• In the tissue phase, leucocytes migrate from the blood into tissues through the process of diapedesis.
• In tissues, they perform their primary defensive functions and survive for a longer duration.
• This phase typically lasts 4 to 5 days or longer, depending on the cell type and physiological conditions.
• Monocytes, after entering tissues, differentiate into macrophages, which are active components of the mononuclear phagocyte system and play a major role in non-specific defence.
• The duration of each phase may vary depending on the type of leucocyte and the presence of infection or inflammation.

Table 17.1: Types of Leucocytes and their distribution in the peripheral blood as DLC and absolute count.

Leucocyte type	Relative proportion (%)	Absolute count (cells/µL)
Neutrophils	50–70	2000–7000
Eosinophils	1–4	40–400
Basophils	0–1	20–100
Monocytes	2–8	200–800
Lymphocytes	20–40	1000–4000

Leukopoiesis

• Leukopoiesis is the process of formation and maturation of leucocytes in the bone marrow.

Stages of Leukopoiesis

• Pluripotent hematopoietic stem cells differentiate into committed progenitor cells.
• Two major lineages arise: myeloid stem cells and lymphoid stem cells.
• Lymphoid stem cells give rise to lymphocytes.
• Myeloid stem cells are trilineage and produce erythroid, megakaryocytic, and granulocyte–monocyte progenitors.
• The granulocyte–monocyte lineage forms specific colony forming units.
• CFU-GM generates precursors for neutrophils and monocytes.
• CFU-G develops into myeloblasts, which mature into neutrophils.
• CFU-M forms monoblasts, which differentiate into monocytes.
• CFU-Eo produces eosinophil precursors that mature into eosinophils.
• CFU-Bas produces basophil precursors that mature into basophils.
• All mature leucocytes arise through sequential stages of blast formation, proliferation, and differentiation.

Steps of Development

• Myeloblasts differentiate into promyelocytes, which represent the next stage in granulocytic maturation.
• Promyelocytes further develop into myelocytes, where specific cytoplasmic granules begin to appear.
• Myelocytes mature into metamyelocytes, showing progressive nuclear indentation and reduced proliferative capacity.
• Metamyelocytes ultimately differentiate into mature leucocytes, including neutrophils, eosinophils, and basophils, which enter the circulation.

Colony-forming Units

• Colony-forming units are progenitor cells derived from committed hematopoietic stem cells.
• Each colony-forming unit is restricted to a particular cell lineage, such as granulocytic, erythroid, or megakaryocytic lines.
• These cells retain limited self-renewal capacity and the ability to proliferate and differentiate into specific blood cell types.
• Colony-forming units cannot be distinguished reliably by routine morphological examination and are identified based on functional assays.
• They play a critical role in regulating hematopoiesis by ensuring a continuous supply of mature blood cells.

Blast Cells

• Blast cells are the earliest morphologically identifiable precursors in leucocyte development, specific for each lineage such as neutrophilic, eosinophilic, basophilic, monocytic, and lymphocytic series.
• They are large cells, approximately 16–20 micrometres in diameter, with a large nucleus.
• The nucleus shows prominent nucleoli, indicating active ribosomal ribonucleic acid synthesis.
• The cytoplasm is scanty and basophilic.
• These cells exhibit high mitotic activity, reflecting rapid proliferation during early hematopoiesis.
• Myeloblasts are earliest precursors of neutrophils, with scanty basophilic cytoplasm and a large nucleus containing multiple nucleoli.
• Distinct precursor forms exist for eosinophilic and basophilic granulocytes.
• Monoblasts give rise to monocytes and resemble myeloblasts, hence termed myelomonocytic precursors.
• Lymphoblasts develop into lymphocytes and show fewer nucleoli with more condensed chromatin.

Promyelocytes and Promonocytes

• Promyelocytes and promonocytes arise from myeloblasts and monoblasts respectively.
• These are large cells with developing granular cytoplasm.
• The nucleus is round, more condensed, and shows fewer nucleoli.
• Cells retain mitotic activity during this stage.
• Promonocytes are larger and mature into monocytes, the largest circulating leucocytes.
• Promyelocytes contain abundant peroxidase-positive granules, about 500 nanometres in diameter.

Neutrophilic Myelocytes

• Neutrophilic myelocytes measure about 12–20 micrometres in diameter.
• The nucleus is round and eccentric, with absent nucleoli.
• Cells undergo limited mitosis, with approximately three divisions at this stage.
• The defining feature is the appearance of specific granules, which are peroxidase negative.
• Granules are spherical or rod shaped, about 200 nanometres in size.
• Primary granules decrease, while secondary granules increase during maturation.

Metamyelocytes

• Metamyelocytes measure about 12–18 micrometres in diameter.
• These cells are nondividing and lack mitotic activity.
• They contain mixed primary and secondary granules during maturation.

Neutrophils

• Neutrophils arise from metamyelocytes, first appearing as band forms.
• These immature cells further differentiate into segmented, mature neutrophils.

Regulation of Leukopoiesis

• Leukopoiesis is regulated by soluble signaling molecules called cytokines, which control proliferation, differentiation, and maturation of white blood cell precursors.
• Major regulators include colony stimulating factors, interleukins, and tumor necrosis factors.
• Hemopoietic growth factors are produced by T lymphocytes, monocytes, fibroblasts, endothelial cells, and organs such as the liver and kidney.
• Interleukins play a key role in lineage-specific development.

1. Interleukin 1, Interleukin 3, and Interleukin 6 stimulate maturation of hematopoietic stem cells.
2. Interleukin 5 promotes differentiation of eosinophils.
3. Interleukin 3 and Interleukin 4 support basophil development.
4. Interleukin 2 suppresses myeloid lineage proliferation, thereby modulating leukopoiesis.

• Colony stimulating factors regulate committed progenitor cells.

1. Granulocyte-monocyte colony stimulating factor stimulates proliferation of pluripotent stem cells and their differentiation into granulocyte and monocyte precursors. It also supports dendritic cell formation in combination with interleukin 4.
2. Granulocyte colony stimulating factor enhances production, maturation, and functional activity of granulocytes.
3. Monocyte colony stimulating factor promotes proliferation and differentiation of monocytes and macrophages and enhances their functional capacity.

• Tumor necrosis factors contribute to regulation by promoting proliferation and differentiation of stem cells, particularly during immune activation and inflammation.
• These factors act in a coordinated and overlapping manner to maintain normal leukocyte levels and to respond rapidly to infection or tissue injury.

Table 17.2: Major hemopoietic growth factors their sources and actions

Growth Factor	Primary Source	Key Biological Action
Interleukin 1	Activated macrophages	Induces acute phase protein synthesis in the liver and stimulates production of other cytokines.
Interleukin 2	T lymphocytes	Promotes proliferation and activation of T lymphocytes.
Interleukin 3	T lymphocytes	Supports survival and expansion of hematopoietic stem cells.
Interleukin 6	T lymphocytes, macrophages	Enhances B and T cell growth and mediates the acute phase response.
Stem cell factor	Stromal cells	Acts synergistically to stimulate pluripotent stem cells.
Granulocyte-monocyte colony stimulating factor	T cells, fibroblasts, endothelial cells	Drives differentiation of granulocytes and monocytes; supports multilineage development.
Granulocyte colony stimulating factor	Macrophages, fibroblasts	Stimulates production and function of neutrophils.
Monocyte colony stimulating factor	Macrophages, endothelial cells	Promotes differentiation and activity of monocytes and macrophages.
Erythropoietin	Kidney, liver	Regulates formation of erythrocytes.
Thrombopoietin	Liver, kidney	Stimulates production of platelets.

Life History of Leukocytes

• The life cycle of leukocytes includes three phases: marrow phase, circulation phase, and tissue phase.

Marrow Phase

• The marrow phase occurs in the bone marrow and represents the developmental stage of leukocytes.
• It consists of two compartments: the mitotic pool and the maturation pool.
• In the mitotic pool, precursor cells develop from myeloblasts to myelocytes through active cell division.
• This phase is characterized by repeated mitosis, which increases the number of precursor cells.
• In the maturation pool, cells differentiate from metamyelocytes into fully mature leukocytes.
• No cell division occurs during this stage, and cellular structures become functionally specialized.
• The total duration of leukocyte development is approximately 10 days.
• About 5 days are spent in the mitotic pool, followed by another 5 days in the maturation pool.
• After maturation, leukocytes enter the bloodstream and later migrate into tissues to perform immune functions.

Circulation Phase

• The circulation phase begins when mature leukocytes are released from bone marrow into the bloodstream.
• Leukocytes remain in circulation for a short duration, usually a few hours, before migrating into tissues.
• Two functional pools exist in blood: the circulating pool and the margination pool.
• In the circulating pool, cells move freely within the bloodstream.
• In the margination pool, especially neutrophils adhere transiently to vascular endothelium.

Tissue Phase

• The tissue phase begins when leukocytes migrate from blood into tissues.
• This forms the tissue pool, where cells perform immune functions.
• Granulocytes survive for a few days and then undergo apoptosis.
• Monocytes persist longer and differentiate into macrophages.
• Tissue macrophages may survive for months to years and provide sustained defense.

Neutrophils

• Neutrophils are the most abundant leukocytes in blood.
• They provide primary defense against acute bacterial infections.
• They exhibit amoeboid movement and migrate by chemotaxis to infection sites.
• They perform phagocytosis and intracellular killing of microbes.
• Neutropenia increases susceptibility to severe bacterial infections.

Structure

• Neutrophils are the most common type of circulating granulocytes.
• Their average diameter ranges from 10 to 14 micrometers.
• The cytoplasm contains fine, pale-staining granules with antimicrobial enzymes.
• The nucleus is typically multilobed, with lobes connected by thin chromatin strands.
• In some cells, a Barr body may be seen as a nuclear appendage.
• Immature neutrophils show a single curved nucleus, known as the band form.
• With maturation, the number of nuclear lobes increases progressively.
• Neutrophils are classified based on nuclear lobes into groups from one to multiple lobes, known as the Arneth classification.
• In normal blood, cells with two to three lobes are most frequent.
• Increased proportion of immature forms indicates a left shift, suggesting active bone marrow response.
• Predominance of highly segmented forms indicates a right shift, often associated with reduced marrow activity.
• Hypersegmented neutrophils are a characteristic finding in megaloblastic anemia due to folate or vitamin B12 deficiency.

Granules of Neutrophils

• Neutrophils contain four types of granules: primary (azurophilic), secondary (specific), tertiary, and secretory granules.
• Primary granules are formed during granulopoiesis, while others develop at later stages of leucopoiesis.

Primary Granules

• These granules are peroxidase-positive and contain myeloperoxidase.
• They include lysosomal enzymes, elastase, proteinases, and antimicrobial proteins such as defensins and cathepsin G.
• Their contents contribute to microbial killing but may also cause tissue damage during inflammation.
• They are reduced or absent in myeloid leukemia and form Auer rods in acute myeloid leukemia.
• Enlarged granules are seen in Chediak–Higashi syndrome.

Secondary Granules

• These granules are peroxidase-negative.
• They contain lactoferrin, lysozyme, gelatinase, and vitamin B12-binding protein.
• Lysozyme and lactoferrin have antimicrobial functions.
• Approximately 16% contain lactoferrin, 24% gelatinase, and 60% both.
• Gelatinase-rich granules are released early during inflammation.
• Abnormalities occur in Pelger–Huët anomaly, Alder–Reilly anomaly, and May–Hegglin anomaly.

Tertiary Granules

• These granules are peroxidase-negative.
• They contain lactoferrin, lysozyme, gelatinase, and vitamin B12-binding protein.
• Lysozyme and lactoferrin have antimicrobial functions.
• Approximately 16% contain lactoferrin, 24% gelatinase, and 60% both.
• Gelatinase-rich granules are released early during inflammation.
• Abnormalities occur in Pelger–Huët anomaly, Alder–Reilly anomaly, and May–Hegglin anomaly.

Secretory Granules

• These are vesicles containing membrane-associated proteins such as receptors (e.g., CD molecules) and enzymes.
• They participate in cell activation and adhesion.
• Toxic granulation is observed in severe infections.

Life History

• Neutrophils pass through four stages: marrow pool, circulating pool, marginating pool, and tissue pool.

Marrow Pool

• The marrow pool represents the developmental and storage phase in bone marrow.
• After maturation from metamyelocytes, neutrophils enter circulation.
• A large reserve of immature cells acts as a readily available source during demand.

Circulation Pool

• Approximately 50% of neutrophils are present in the circulating pool and actively flow in blood.
• These cells are measured in routine blood counts.

Margination Pool

• The remaining 50% adhere to the endothelial lining of blood vessels.
• These cells can rapidly enter circulation when needed.
• Loss of margination leads to transient neutrophilia.

Tissue Pool

• Neutrophils circulate for 6–8 hours before migrating into tissues.
• In tissues, they survive for about 4 days and perform phagocytic functions.

Neutrophil Count

1. Neutrophils constitute approximately 50–70% of total leukocytes in peripheral blood.
2. Their count is regulated by a balance between neutrophilopoiesis and distribution into circulating, marginating, and tissue pools.
3. In circulation, many neutrophils undergo margination, adhering to vascular endothelium.
4. This adhesion is mediated by selectins, which are carbohydrate-binding molecules.
5. Temporary release from the marginating pool can increase circulating neutrophil count.
6. The neutrophil count varies in different physiological and pathological conditions.

Table 17.3: Conditions that alter neutrophil count

Feature	Great (Long) Saphenous Vein	Small (Short) Saphenous Vein
Formation	Formed by the union of the medial end of the dorsal venous arch and the medial marginal vein of the foot	Formed by the union of the lateral end of the dorsal venous arch and the lateral marginal vein of the foot
Relation to the malleolus	Passes anterior to the medial malleolus	Passes posterior to the lateral malleolus
Number of valves	Usually 10–20 valves	Usually 7–12 valves
Related sensory nerve	Saphenous nerve	Sural nerve
Termination	Drains into the femoral vein at the saphenofemoral junction	Drains into the popliteal vein in the popliteal fossa

Functions of Neutrophil

• Neutrophils are highly phagocytic cells that ingest and destroy microorganisms.
• They contain antimicrobial and bactericidal substances within their granules.
• They are the first line of defense against acute bacterial (pyogenic) infections.
• Neutropenia increases susceptibility to pyogenic infections.

Neutrophil Phagocytosis

• Phagocytosis involves ingestion and destruction of microbes by neutrophils, monocytes, and macrophages.
• The sequence includes chemotaxis, diapedesis, adherence, ingestion, and killing.

Chemotaxis

• Chemotaxis is the directed migration of neutrophils toward the site of infection.
• It is mediated by chemotactic factors released from microbes, leukocytes, and damaged tissues.
• Complement components (C3a, C5a) act as important chemoattractants.
• Neutrophils become ameboid and bone marrow increases production during this phase.

Diapedesis

• Diapedesis is the movement of neutrophils from blood into tissues.
• It involves margination, rolling, adhesion, and passage through endothelium.
• This process is mediated by selectins and adhesion molecules.

Opsonization and Adherence

• Opsonization enhances phagocytosis by coating microbes with opsonins.
• Major opsonins include IgG and complement (C3b).
• This facilitates adherence of microbes to neutrophil receptors.

Ingestion

• Neutrophils extend pseudopodia to surround microbes.
• The engulfed particle forms a phagosome, which fuses with lysosomes to form a phagolysosome.

Killing Mechanisms

Nonoxidative Mechanisms

• Granules contain lysozyme, lactoferrin, proteases, and defensins.
• These substances destroy bacterial cell walls and inhibit growth.

Oxidative Mechanisms

• Activated neutrophils generate reactive oxygen species (ROS) such as O₂⁻, H₂O₂, OH·, and HOCl.
• These are produced via NADPH oxidase during the respiratory burst.
• Superoxide dismutase (SOD) converts O₂⁻ to H₂O₂.
• Myeloperoxidase converts Cl⁻ to HOCl, a potent antimicrobial agent.

Interaction of Mechanisms

• Nonoxidative enzymes enhance oxidative killing by creating a localized destructive environment.
• Excess activity may damage host tissues, as seen in rheumatoid arthritis.

Eosinophils

• Eosinophils play an important role in allergic reactions (including asthma) and helminthic infections.
• They release proinflammatory mediators similar to neutrophils.
• Their granules contain cytotoxic proteins that damage parasites and host cells.
• They also produce mediators such as leukotrienes and platelet-activating factor (PAF).
• Eosinophils have a short lifespan in blood but survive longer in tissues.
• They are predominantly tissue-resident cells, with an approximate ratio of 100:1 (tissue: blood).
• They are found in the epithelium of respiratory, gastrointestinal, and genitourinary tracts.
• Their production and activity are regulated mainly by interleukin-5 (IL-5).

Structure

• Eosinophils are granular leukocytes similar in size to neutrophils.
• Their cytoplasm contains coarse, brick-red granules on Leishman staining.
• The nucleus is typically bilobed, with lobes connected by a thick strand, giving a spectacle-like appearance.

Chemicals Secreted by Eosinophils

• Eosinophils release multiple chemical mediators involved in immune and inflammatory responses.
• These substances are classified into three groups:
  • Granule-derived proteins
  • Cytokines
  • Lipid-derived mediators
• Granule proteins are specific to eosinophils and contribute to cytotoxic and defense functions.
• Cytokines and lipid mediators are also produced by other inflammatory cells.
• Together, these mediators regulate allergic reactions, parasitic defense, and inflammatory processes.

Table 17.4: Eosinophil-derived chemicals

Category	Substances
Granule-derived proteins	Major basic protein, eosinophil cationic protein, eosinophil peroxidase, eosinophil-derived neurotoxin
	Lysophospholipase, phospholipase D, arylsulfatase, acid phosphatase
	Catalase, histaminase, hexosaminidase
Cytokines (stored or newly synthesized)	GM-CSF, TGF-α, TGF-β, macrophage migration inhibitory factor (MIF)
	Interleukins (IL-1 to IL-6, IL-8, IL-12)
	Tumor necrosis factor-α (TNF-α)
Lipid-derived mediators	Leukotriene C, platelet-activating factor (PAF)
	Prostaglandins (PGE₁, PGE₂)
	Thromboxane B₂

Eosinophil Granular Contents

• Eosinophil granules contain multiple bioactive substances, including major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil peroxidase (EP), eosinophil-derived neurotoxin (EDN), cytokines, and other enzymes.

Major Basic Protein

• MBP is the principal cytotoxic protein of eosinophils.
• It damages helminths and larvae, including Schistosoma mansoni, Ancylostoma duodenale, Ascaris lumbricoides, Toxocara canis, Wuchereria bancrofti, Trichinella spiralis, and Strongyloides stercoralis.
• Eosinophils bind to IgG-coated parasites and release MBP onto their surface.
• MBP also injures respiratory epithelium and increases bronchial hyperreactivity, contributing to asthma.
• It activates platelets, basophils, and mast cells.

Eosinophil Cationic Protein

• ECP is an arginine-rich protein with cytotoxic activity.
• It is harmful to helminths and respiratory epithelium.
• Along with EDN, it contributes to neurotoxicity.

Eosinophil Peroxidase

• EP is a heme-containing enzyme structurally similar (68%) to myeloperoxidase.
• It is toxic to parasites and lung tissue, and its activity increases in the presence of H₂O₂.

Eosinophil-derived Neurotoxin

• EDN is a glycoprotein with strong ribonuclease activity.
• It is toxic to neural tissue but has limited effect on non-neural parasitic tissue.

Cytokines

• Cytokines are synthesized in the cytoplasm and subsequently stored within granules for release.
• Important cytokines secreted by eosinophils include:
  • Interleukins 1–6, 8, and 12
  • Granulocyte–macrophage colony-stimulating factor
  • Macrophage migration inhibitory factor
  • Transforming growth factors alpha and beta
  • Tumor necrosis factor alpha
• Transforming growth factor alpha contributes to tissue repair and wound healing.
• Certain cytokines, including interleukin 5, interleukin 3, and granulocyte–macrophage colony-stimulating factor, act in an autocrine manner to promote eosinophil growth and activation.
• Macrophage migration inhibitory factor is involved in the pathogenesis of acute respiratory distress syndrome.

Other Chemicals

• Lysophospholipase constitutes about 10% of eosinophil proteins.
• Additional enzymes include phospholipase D, arylsulfatase-B, acid phosphatase, catalase, and histaminase.
• These enzymes participate in inflammation and parasite destruction.

Functions of Eosinophils

• Eosinophils migrate from blood into tissues through chemotaxis and endothelial adhesion.
• Migration is guided by specific chemoattractants such as eosinophil chemotactic factor of anaphylaxis (ECF-A), a tetrapeptide that promotes tissue accumulation.
• They are mainly involved in:

1. Defense against helminthic infections
2. Regulation of allergic reactions

In Helminthic Infections

• Eosinophils act against parasites such as Schistosoma mansoni, Trichinella spiralis, Toxocara canis, Necator brasiliensis, Fasciola hepatica, and other roundworms and hookworms.
• They bind to larvae coated with immunoglobulin G, immunoglobulin E, and complement proteins.
• This binding activates eosinophils, leading to degranulation within about 3 hours.
• Released cytotoxic proteins damage the parasite surface (tegument).
• Eosinophils then penetrate beneath the tegument and destroy larval tissue, followed by phagocytosis of debris.

In Allergy

• Eosinophils contribute to bronchial asthma and allergic inflammation.
• Increased eosinophil count correlates with bronchial hyperresponsiveness.
• Glucocorticoids reduce eosinophil numbers and improve symptoms by inhibiting interleukin 5 and interleukin 3 production from type 2 helper lymphocytes.
• Recruitment of eosinophils is associated with increased activated T cells and monocytes.
• Asthma is considered a type 2 helper lymphocyte-driven, eosinophil-mediated disease.

Alterations in Eosinophil Count

• Normal eosinophils constitute 2–4% of leukocytes or 40–440 per μL of blood.
• Increased count is termed eosinophilia, while decreased count is termed eosinopenia.

Table 17.5: Causes of eosinophilia and eosinopenia

Condition	Category	Examples / Causes
Eosinophilia	Parasitic (helminthic) infections	Ascariasis, filariasis, toxocariasis, ancylostomiasis (hookworm), strongyloidiasis, trichinosis, schistosomiasis, fascioliasis, echinococcosis
	Allergic disorders	Bronchial asthma, allergic rhinitis, atopic dermatitis, urticaria, food allergy, hay fever
	Drug-related	Drug hypersensitivity reactions
	Hematological disorders	Eosinophilic leukemia
	Pulmonary condition	Tropical pulmonary eosinophilia
	Endocrine disorder	Addison’s disease
	Miscellaneous	Eosinophilia–myalgia syndrome
Eosinopenia	Hormonal / therapy-related	Glucocorticoid therapy, Cushing’s syndrome
	Bone marrow disorders	Aplastic anemia
	Drug-induced	Agranulocytosis due to drugs

Table 17.6: Differences between basophils and mast cells.

Feature	Basophils	Mast Cells
Origin	Derived from bone marrow	Derived from bone marrow
Presence in blood	Present in circulation	Absent in circulation
Location in tissues	Not normally resident in connective tissue	Primarily located in connective tissue
Life span	Survive for a few days	Survive for several months
Growth factors	Stimulated mainly by interleukin-3	Dependent on stem cell factor
Major secretions	Release histamine and interleukin-4	Release histamine and heparin
Receptors	Possess receptors for immunoglobulin E and G	Possess receptors mainly for immunoglobulin E

Basophils And Mast Cells

• Basophils are the least abundant granulocytes, comprising <0.5% of leukocytes in peripheral blood.
• They circulate in blood and migrate into tissues during inflammatory and immune responses, but do not normally reside there.
• Mast cells originate from bone marrow precursors and primarily reside in connective tissue, especially beneath epithelia and around blood vessels, with a longer lifespan.
• Both cells express high-affinity immunoglobulin E (IgE) receptors.
• They play a central role in allergic (type I hypersensitivity) reactions.
• These cells also contribute to host defense mechanisms associated with immunoglobulin E-mediated immunity.

Structure

Basophils

• Basophils measure approximately 10–14 μm, similar to neutrophils.
• The nucleus is poorly segmented, often appearing S-shaped, with dense chromatin.
• Cytoplasm contains large, numerous granules that may obscure the nucleus.
• Granules are membrane-bound and may contain Charcot–Leyden crystal proteins.
• The cytoplasm includes glycogen, mitochondria, free ribosomes, and few lipid bodies.
• Granules release mediators such as histamine, chondroitin sulfate, leukotrienes, tryptase, and eosinophil chemotactic factor of anaphylaxis.

Mast Cells

• Mast cells are round or elongated tissue cells with a non-segmented nucleus.
• They contain numerous smaller granules and abundant cytoplasmic filaments and lipid bodies, but lack glycogen.
• Granules release histamine, heparin, chondroitin sulfate, proteases, and chemotactic factors.
• Two types are recognized: mucosal mast cells and connective tissue mast cells.

Differences between Basophils and Mast Cells

• Although basophils and mast cells share functional similarities, they differ in location, structure, and lifespan.

Functions

• Basophils and mast cells are primarily involved in allergic and inflammatory responses.
• Degranulation occurs when an allergen cross-links immunoglobulin E (IgE) bound to high-affinity Fc receptors on these cells.
• This leads to release of mediators such as histamine, which produce both protective and tissue-damaging effects.

Role in Acute Allergic Reactions

• IgE-mediated activation triggers rapid degranulation and histamine release.
• This mechanism underlies acute conditions such as allergic rhinitis, urticaria, and food allergy.
• Excessive mediator release results in anaphylaxis, a severe systemic hypersensitivity reaction.
• In anaphylaxis, histamine causes vasodilation, increased vascular permeability, and reduced cardiac output, leading to acute hypotension.

Role in Chronic Allergic Reactions

• These cells contribute to late-phase allergic responses, especially in bronchial asthma.
• Mast cells release cytokines that recruit eosinophils, basophils, and other leukocytes.
• This process, termed the mast cell–leukocyte cascade, sustains chronic inflammation.

Role in T-cell Dependent Responses

• T lymphocytes secrete growth factors that enhance basophil and mast cell activation.
• These cells participate in cell-mediated immune reactions within tissues.

Role in Host Defense

• Basophils and mast cells contribute to defense against viral, bacterial, and parasitic infections.
• Their numbers may increase in infections such as chickenpox, smallpox, and tuberculosis.

Normal Count

• Basophils constitute 0–1% of leukocytes, with an absolute count of 20–80/μL.
• Mast cells are not normally present in circulating blood.
• Increased basophil count is termed basophilia, and decreased count is basopenia.

Mastocytosis

• Secondary mast cell increase occurs in allergic and connective tissue disorders such as asthma and rheumatoid arthritis.
• Primary proliferation leads to systemic mastocytosis, classified as:
  • Category I: Indolent form (e.g., urticaria pigmentosa)
  • Category II: Associated with myeloproliferative disorders
  • Category III: Aggressive form with lymphadenopathy and eosinophilia
  • Category IV: Mast cell leukemia

Table 17.7: Alteration in basophil count

Condition	Category	Examples / Causes
Basophilia	Allergic and inflammatory disorders	Ulcerative colitis, erythroderma, urticaria, drug and food hypersensitivity
	Infections	Chickenpox, smallpox, influenza, tuberculosis
	Endocrine disorders	Myxedema (hypothyroidism), diabetes mellitus
	Nutritional deficiency	Iron deficiency
	Hematological disorders	Basophilic leukemia, polycythemia
Basopenia	Hormonal / therapy-related	Cortisol therapy, Cushing’s syndrome
	Endocrine disorder	Hyperthyroidism
	Physiological condition	Ovulation
	Hypersensitivity states	Certain allergic reactions

Monocytes And Macrophages

• Monocytes are the largest leukocytes present in peripheral blood.
• After circulating, they migrate into tissues and differentiate into macrophages.
• Both are mononuclear phagocytes involved in nonspecific immune defense.
• Tissue macrophages constitute the mononuclear phagocyte system (previously termed reticuloendothelial system).
• They play a key role in phagocytosis and microbial clearance.
• Monocytes act as the second line of defense against infections.

Monocytes

Morphology

• Monocytes are the largest leukocytes, measuring 12–25 μm in diameter.
• The nucleus occupies about 50% of the cell and is usually eccentric.
• It is commonly kidney-shaped (reniform), but may also appear horseshoe-shaped, round, or irregular.
• Nuclear chromatin forms a delicate, irregular network, giving a non-homogeneous appearance.
• The cytoplasm is abundant and has a ground-glass appearance.
• Although classified as agranular cells, about 40% contain fine granules, and occasional azurophilic granules are present.
• Granules contain hydrolytic enzymes such as acid phosphatase and lysozyme, while alkaline phosphatase is absent.
• Surface receptors include those for immunoglobulins (IgG, IgA, IgE), complement, cytokines, and certain hormones.
• They express major histocompatibility complex class II, enabling antigen presentation.

Functions

• Phagocytosis and microbial killing:

1. Monocytes exhibit chemotaxis and motility.
2. Surface receptors enhance recognition of immunoglobulins, complement, and microbial components.
3. Microbes are destroyed by reactive oxygen species, nitric oxide, and lysosomal enzymes.
4. They are effective against intracellular pathogens, including viruses and parasites.

• Antigen presentation:

1. Processed antigens combine with MHC class II molecules and are displayed on the cell surface.
2. This activates T lymphocytes and initiates cell-mediated immunity.

• Cytokine and mediator release:

1. Monocytes secrete interleukin-1, interleukin-6, tumor necrosis factor alpha, and interferons.
2. They produce growth factors such as granulocyte–macrophage colony-stimulating factor, macrophage colony-stimulating factor, transforming growth factor, platelet-derived growth factor, and fibroblast growth factor.
3. They also release complement proteins and enzymes like collagenase and elastase, contributing to inflammation and tissue repair.

Life Span

• In circulation, monocytes have a half-life of 10–72 hours.
• After entering tissues, they differentiate into macrophages and survive for weeks to months (average about 3 months).
• During inflammation, they accumulate within hours, and after 12 hours, they may outnumber neutrophils.

Normal Count

• Monocytes constitute 2–8% of leukocytes, with an average absolute count of 400/μL.
• Counts are higher in neonates and infants (~1000/μL).
• A count exceeding 800/μL in adults is termed monocytosis, while reduced levels are called monocytopenia.

Macrophages

• Monocytes migrate into tissues and differentiate into macrophages, forming the mononuclear phagocyte system.
• Resident macrophages are capable of self-renewal by cell division.
• Differentiation is associated with increased cell size, cytoplasmic granules, vacuoles, and variability in shape.
• The average size ranges from 25–50 μm.
• The nucleus is eccentric, usually kidney-shaped or spindle-shaped, with one or two nucleoli.
• The cytoplasm contains numerous azurophilic granules.
• Macrophages retain surface receptors similar to monocytes.
• In chronic inflammation, they may fuse to form multinucleated giant cells with enhanced phagocytic activity.

Dendritic Cells

• Some monocytes differentiate into dendritic cells, which are specialized antigen-presenting cells.
• They are highly efficient in antigen capture and presentation to T lymphocytes.
• Unlike macrophages, they have limited phagocytic capacity.
• They lack receptors for immunoglobulins, complement, and colony-stimulating factors, and have no prominent granules.
• They constitute about 0.1–1% of mononuclear cells in blood and bone marrow.
• They are present as Langerhans cells in skin, interdigitating cells in thymus, and interstitial cells in organs such as lung and heart.

Table 17.8: Causes of alteration in monocyte count.

Condition	Category	Examples / Causes
Monocytopenia	Bone marrow suppression	Aplastic anemia
	Hematological malignancy	Hairy cell leukemia
	Severe infections	Septicemia
Monocytosis	Leukemias	Acute monocytic leukemia, chronic myelomonocytic leukemia
	Lymphoproliferative disorder	Hodgkin’s disease
	Myeloproliferative disorder	Polycythemia vera
	Hemolytic condition	Hemolytic anemia
	Post-surgical state	Post-splenectomy
	Viral infection	Cytomegalovirus infection
	Autoimmune disorders	Collagen vascular diseases
	Parasitic infections	Malaria, kala-azar
	Drug-related	Glucocorticoid therapy
	Idiopathic	Chronic idiopathic monocytosis

Table 17.9: Distributions of cells of mononuclear phagocyte system

Location	Cell Types
Blood	Monocytes
Bone marrow	Monoblasts, promonocytes
Tissues	Kupffer cells (liver), osteoclasts (bone), alveolar macrophages (lungs)
	Histiocytes (connective tissue), microglia (brain)
	Red pulp macrophages (spleen), macrophages (lymph nodes and thymus)
	Mesangial cells (kidney), dendritic cells / histiocytes (skin)
	Type A synovial cells (synovium)
Body cavities	Pleural macrophages, peritoneal macrophages
Inflammatory sites	Epithelioid cells, multinucleated giant cells

Lymphocytes

• Lymphocytes are a heterogeneous group of leukocytes with distinct structural and functional types.
• They are classified morphologically into small and large lymphocytes.

Structure

Small Lymphocytes

• Small lymphocytes measure 6–9 μm, approximately the size of red blood cells.
• They form about 20–50% (average ~35%) of circulating lymphocytes.
• The nucleus is oval or kidney-shaped, densely packed with chromatin, and occupies nearly 90% of the cell.
• The cytoplasm appears as a thin rim and lacks visible granules.

Large Lymphocytes

• Large lymphocytes measure 10–15 μm and constitute 50–80% (average ~65%) of lymphocytes.
• The nucleus is oval or reniform, compact, and eccentrically located.
• The cytoplasm is more abundant and deep blue, usually without granules.
• About 3% contain coarse cytoplasmic granules (5–15 per cell), representing natural killer or T lymphocytes.

Normal Count and Functions

• Lymphocytes account for 20–40% of total leukocytes, with an absolute count of 500–8000 per mm³ of blood.
• Increased count is termed lymphocytosis, and decreased count is lymphocytopenia.
• Functionally, lymphocytes are classified as:

1. B cells, which differentiate into plasma cells and produce antibodies (humoral immunity)
2. T cells, which mediate cell-mediated immunity
3. Natural killer cells, which provide innate, nonspecific defense

Table 17.10: Causes of lymphocytosis and lymphocytopenia.

A. Lymphocytosis
1. Primary lymphocytosis
– Acute lymphocytic leukemia
– Chronic lymphocytic leukemia
– Adult T-cell leukemia
– NK-cell leukemia
– Monoclonal B-cell lymphocytosis
2. Reactive lymphocytosis
– Infectious mononucleosis
– Bordetella pertussis
– Tuberculosis
– Postsplenectomy
– Cigarette smoking
– Septic shock
– Drugs
B. Lymphocytopenia
1. Acquired lymphocytopenia
– Aplastic anemia
– AIDS
– Hepatitis
– Glucocorticoid therapy
– Typhoid fever
– Systemic lupus erythematosus
2. Inherited lymphocytopenia
– Severe combined immunodeficiency states
– Wiskott-Aldrich syndrome
– Immunodeficiency with thymoma
– Cellular immunodeficiency with immunoglobulins

Table 17.11: Summary of the formed elements in blood.

Name	Count	Features	Functions
A. Red blood cells (erythrocytes)	5.4 million/mm3 in males; 4.8 million/mm3 in females	7–8 μm diameter, biconcave discs, without a nucleus, life span about 120 days	Transport oxygen and carbon dioxide
B. White blood cells (leukocytes)	4000–11,000/mm3	Live for a few hours to few days	Kill pathogens (body defence)
Neutrophils	50–70% of all WBCs	10–14 μm diameter; nucleus is multilobed, connected by thin strands of chromatin; cytoplasm has fine, pink granules	Phagocytosis of organisms (first line of defence)
Eosinophils	1–4% of all WBCs	10–14 μm diameter; nucleus is bilobed; coarse brick-red granules in cytoplasm	Combat the effects of histamine in allergic reactions, kill parasitic worms
Basophils	0–1% of all WBCs	10–14 μm diameter; nucleus is bilobed or irregular in shape; Large cytoplasmic granules are deep blue-purple	Release heparin, histamine, and serotonin in allergic reactions that promote overall inflammatory response
Lymphocytes	20–40% of all WBCs	Small lymphocytes are 6–9 μm in diameter; large lymphocytes are 10–14 μm in diameter;	Mediate immune responses
Monocytes	2–8% of all WBCs	nucleus is round or slightly indented; cytoplasm forms a clear rim around the nucleus	Phagocytosis (after transforming into tissue macrophages)
A. Platelets (thrombocytes)	150,000–400,000/mm3	2–4 μm diameter, cell fragments, no nucleus	Form platelet plug in hemostasis (temporary hemostatic plug)

Leukemia

Definition and Concept

• Leukemia is a malignant disorder of hematopoietic cells characterized by uncontrolled proliferation of leukocytes and their precursors.
• It leads to the presence of immature and abnormal cells in peripheral blood, often with marked leukocytosis.
• There is extensive infiltration of bone marrow and other tissues by malignant cells.
• Total leukocyte count is usually very high, except in subleukemic or aleukemic forms.
• Although primarily affecting the leukocytic series, involvement of erythroid precursors or megakaryocytes may occur.

Types

• Leukemia is classified into:

1. Myeloid leukemia
2. Lymphocytic leukemia

• Each type is further divided into acute and chronic forms.

Acute Leukemias

Acute Lymphoblastic Leukemia

• ALL commonly affects children and young adults, accounting for about 80% of childhood leukemias.
• Patients present with anemia, bleeding, infections, fever, and malaise.
• Lymphadenopathy, hepatomegaly, and splenomegaly are frequent.
• Blood findings include anemia, thrombocytopenia, and increased leukocyte count with predominant lymphoblasts.

Acute Myeloblastic Leukemia

• AML mainly occurs in adults aged 15–40 years and forms about 20% of childhood cases.
• Clinical features resemble ALL, but organ enlargement is less prominent.
• Blood shows anemia, thrombocytopenia, and elevated leukocyte count with >60% myeloblasts.

Chronic Leukemias

Chronic Myeloid Leukemia

• CML accounts for about 20% of leukemias and is common in adults aged 30–60 years.
• Onset is gradual, with fatigue, weight loss, and anemia.
• Splenomegaly is a prominent feature; hepatomegaly may occur.
• Leukocyte count often exceeds 100,000/mm³, with predominance of neutrophils and their precursors.
• Blast cells are few, except during blast crisis.

Chronic Lymphocytic Leukemia

• CLL is a slowly progressive leukemia seen mainly in individuals >50 years, with higher incidence in males.
• Patients present with mild symptoms and prominent lymphadenopathy.
• Hepatosplenomegaly may be present.
• Leukocyte count is increased, with >90% mature lymphocytes.