# White Blood Cells (Leukocytes) – Types, Structure, Functions & WBC Count

> Learn what white blood cells (leukocytes) are, how the five major types differ, where they develop, how they protect the body, and what WBC counts mean.

Canonical URL: https://biologynotesonline.com/white-blood-cell-leukocytes/
Author: Sourav Pan
Last updated: September 6, 2026

![White Blood Cells (Leukocytes) – Types, Structure, Functions & WBC Count](https://biologynotesonline.com/wp-content/uploads/2023/09/White-Blood-Cell-Leukocytes-Definition-Types-Structure-Functions.jpg)

White blood cells (leukocytes) protect the body from infections, abnormal cells, and other biological threats. They develop from hematopoietic precursors and include neutrophils, eosinophils, basophils, monocytes, and lymphocytes, each with specialized roles in innate or adaptive immunity. Leukocytes circulate in blood but can also migrate into tissues where immune defense is needed.

## What Are White Blood Cells (Leukocytes)?

Leukocytes, also called white blood cells (WBCs), are nucleated cells of the immune system that protect the body against infections and other harmful agents. These cells do not remain only in blood. They circulate in the bloodstream, pass through lymphatic tissues and many leukocytes move out of blood vessels into different body tissues where immune functions are carried out. 

The term “white” does not mean that individual WBCs actually appear white under microscope. Unstained leukocytes are colorless, while their nucleus and other cellular structures become clearly visible after suitable staining. 

In centrifuged blood, leukocytes together with platelets form the pale buffy coat. This is fundamentally different from [erythrocytes (red blood cells)](https://biologynotesonline.com/red-blood-cell/), which are mainly used for transport of oxygen and carbon dioxide by hemoglobin. 

Leukocytes are mainly defense cells, and can leave the blood to perform their functions in tissues.

![White Blood Cell (Leukocytes)
](https://biologynotesonline.com/wp-content/uploads/2024/04/image-1175.png)White Blood Cell (Leukocytes)

## Structural Features That Distinguish Leukocytes from Other Blood Cells

Leukocytes (WBCs) are complete nucleated blood cells having cytoplasm and different cellular organelles. Their structure is quite different from erythrocytes and platelets. The shape, size, nucleus and amount of cytoplasm are also not same in all WBCs. Some contain numerous granules while in others these are less prominent.

The following are the major structural features of leukocytes-

- Nucleus- A well-developed nucleus is present in leukocytes. It may be round, indented, kidney-shaped, bilobed or divided into several lobes depending on the cell. After staining, the nucleus can be clearly observed. Mature mammalian erythrocytes, on the other hand, do not contain a nucleus.

- Cell membrane and shape- The leukocyte is surrounded by a plasma membrane. WBCs do not have a fixed biconcave disc-like shape as seen in erythrocytes. Their shape can be changed during movement. The cell membrane can also extend out during amoeboid movement, and the cells can squeeze between endothelial cells of blood vessels. This process is referred to as diapedesis (emigration).

- Cytoplasm- Cytoplasm is present around the nucleus and its amount varies among different WBCs. In some leukocytes, only a thin layer of cytoplasm can be seen around a large nucleus. Other cells contain much more cytoplasm. Different organelles and intracellular components are present within it.

- Cytoplasmic granules- Many WBCs have granules within their cytoplasm. In some cells these granules are numerous and clearly visible, whereas in others the specific granules are absent or are not easily seen under routine light microscope. Azurophilic granules (lysosomal granules) are also found in different leukocytes. Depending on their staining property, the granules may appear pink, reddish, blue-purple or comparatively pale.

- Cellular organelles- Leukocytes retain their intracellular organelles and remain complete functioning cells. Lysosomal structures are especially developed in several WBCs which take part in breakdown of engulfed materials. Mature erythrocytes are different. During maturation they lose the nucleus, mitochondria, endoplasmic reticulum and most of the other internal components.

- Size- WBCs have a variable size. Small leukocytes may measure around 6–9 µm, many granular WBCs are about 12–15 µm, while larger forms may reach approximately 12–20 µm or more. Most of them are therefore larger than erythrocytes (about 7–8 µm). The smallest leukocytes, however, may be almost similar in size to RBCs.

- Absence of erythrocyte-type structure- Leukocytes are not filled with hemoglobin and they do not possess the regular biconcave structure of erythrocytes. RBC structure is mainly adapted for the transport of respiratory gases. WBCs retain nucleus, cytoplasm and organelles for carrying out different immune cellular activities.

Difference from platelets- A WBC is a complete cell. [Platelets](https://biologynotesonline.com/platelets/) are different, they are small membrane-covered fragments of cytoplasm released from megakaryocytes. They are generally about 2–4 µm in diameter and do not contain a nucleus.

## Leukopoiesis: Myeloid and Lymphoid Development

Leukopoiesis is the process by which white blood cells are formed from [hematopoietic stem cells (HSCs)](https://biologynotesonline.com/hematopoiesis-definition-process-locations/). In adults, the process mainly takes place in bone marrow. HSCs first form multipotent progenitors, and during further development the cells become committed towards myeloid or lymphoid lineage. T-cell precursor is different. It leaves the bone marrow and major maturation takes place in the thymus.

![Hematopoietic stem cell differentiation into myeloid and lymphoid pathways producing granulocytes, monocytes, B cells, T cells, and NK cells.](https://biologynotesonline.com/wp-content/uploads/2024/04/Leukopoiesis-and-White-Blood-Cell-Development-819x1024.webp)Hematopoietic stem cell differentiation into myeloid and lymphoid pathways producing granulocytes, monocytes, B cells, T cells, and NK cells.

The following are the main steps involved in leukopoiesis-

### 1. Formation of Multipotent Progenitor

Hematopoietic stem cells (HSCs) are self-renewing cells from which different blood cell lineages are produced. In the first step, HSC gives rise to multipotent progenitor (MPP). The self-renewing property is gradually reduced in these cells, and their developmental ability becomes more restricted.

HSC → Multipotent progenitor (MPP)

### 2. Myeloid and Lymphoid Lineage Separation

From multipotent progenitor, two major groups are formed in the classical pathway. These are common myeloid progenitor (CMP) and common lymphoid progenitor (CLP).

CMP gives rise to the cells of myeloid lineage. CLP, on the other hand, produces the lymphoid cells.

MPP → CMP → Myeloid lineage

MPP → CLP → Lymphoid lineage

### 3. Formation of Myeloid Progenitor

In this step, common myeloid progenitor (CMP) further develops towards granulocyte-monocyte progenitor (GMP). The GMP has a more restricted developmental capacity and produces mainly granulocytes and monocytes.

Another branch of CMP is the megakaryocyte-erythroid progenitor. It forms erythrocytes and platelets. Hence, this branch does not form leukocytes.

HSC → MPP → CMP → GMP → Granulocytes and monocytes

### 4. Development of Granulocytes

Granulocyte-committed precursor cells are produced from the myeloid progenitors. In neutrophil development, myeloblast is the first recognizable precursor. Myeloblast changes into promyelocyte, followed by myelocyte. These cells still have the capacity to divide.

After the myelocyte stage, metamyelocyte is formed. It then develops into a band cell and finally a mature segmented neutrophil. Cell division stops around the metamyelocyte stage.

During this process, the cytoplasmic granules also appear stepwise. Primary (azurophilic) granules are formed early, while specific granules become more prominent during the myelocyte stages. The nucleus also undergoes changes. It changes from round form to indented, then band-shaped and finally becomes segmented.

GMP → Myeloblast → Promyelocyte → Myelocyte → Metamyelocyte → Band cell → Mature neutrophil

### 5. Eosinophil and Basophil Development

Eosinophils are also derived from the myeloid progenitors. An eosinophil-committed progenitor (EoP) is formed during this pathway. Interleukin-5 (IL-5) has an important role in their expansion and maturation, together with IL-3 and GM-CSF.

Basophils develop through basophil-directed myeloid progenitors. Their development is strongly supported by IL-3, particularly during immune responses.

### 6. Development of Monocytes

The monocytic cells are formed through the myeloid pathway. A committed precursor first gives rise to monoblast. Monoblast then changes into promonocyte and finally the mature monocyte.

GMP → Monocyte progenitor → Monoblast → Promonocyte → Monocyte

The mature monocytes are released into blood. After moving into tissues, many of these cells can further develop into macrophages or monocyte-derived dendritic cells depending upon tissue and inflammatory conditions.

### 7. Formation of Lymphoid Progenitor

In lymphoid development, the multipotent progenitors first become lymphoid-primed progenitors. These then form common lymphoid progenitors (CLPs). At this stage, the developmental ability becomes more restricted.

In the classical pathway, CLPs can give rise to B lymphocytes, T lymphocytes and natural killer (NK) cells.

HSC → MPP → Lymphoid-primed progenitor → CLP → B cells, T cells and NK cells

### 8. B-Lymphocyte Development

Most of the early B-cell development takes place in bone marrow. The CLP-derived cells first form pro-B cells. These are then changed into pre-B cells and immature B cells.

During these stages, immunoglobulin gene rearrangement takes place. A functional B-cell receptor is gradually formed. The immature B cell then proceeds towards mature B lymphocyte.

CLP → Pro-B cell → Pre-B cell → Immature B cell → Mature B lymphocyte

### 9. T-Lymphocyte Development

T-cell precursors originate from the bone marrow progenitors, but their major maturation does not take place there. The precursor cells enter blood and move into the thymus.

In thymus, these cells pass through different thymocyte stages. Antigen-receptor gene rearrangement and selection takes place during this process. Finally, mature T lymphocytes are produced.

CLP-derived precursor → Thymic precursor → Thymocyte stages → Mature T lymphocyte

### 10. NK-Cell Development

Natural killer (NK) cells are derived from lymphoid progenitors. CLP-derived cells pass through NK-restricted progenitor stages and gradually acquire the receptors and cellular characters of mature NK cells. Much of this development takes place in bone marrow.

### 11. Maturation and Release of Leukocytes

After the required maturation, leukocytes are released into blood circulation or enter the lymphoid tissues. Different cytokines and growth factors control their production. G-CSF mainly supports neutrophil formation. M-CSF supports monocytic development, while IL-5 has an important role in eosinophil development. IL-7 supports different stages of lymphocyte development.

## Classification of Leukocytes by Morphology and Cell Lineage

Leukocytes (WBCs) are classified based on their morphology and the developmental lineage from which these cells are formed. Morphologically, they are divided into granulocytes and agranulocytes. According to cell lineage, the major groups are myeloid and lymphoid cells. These two classifications are not exactly the same. Monocyte, for example, is an agranulocyte based on morphology but is developed through the myeloid lineage.

![White blood cells classified as granulocytes or agranulocytes by morphology and as myeloid or lymphoid cells by developmental lineage.](https://biologynotesonline.com/wp-content/uploads/2024/04/Classification-of-Leukocytes-by-Morphology-and-Cell-Lineage-819x1024.webp)White blood cells classified as granulocytes or agranulocytes by morphology and as myeloid or lymphoid cells by developmental lineage.

### A. Classification Based on Morphology

The morphological classification of leukocytes is based mainly on the presence of visible cytoplasmic granules and appearance of nucleus in a stained blood smear. The following are the two groups-

### 1. Granulocytes

Granulocytes are WBCs which possess prominent specific granules in their cytoplasm. The nucleus is generally divided into lobes. Because of this appearance, these cells are also referred to as polymorphonuclear leukocytes.

Three types are present, neutrophils, eosinophils and basophils.

a. Neutrophils -contain a multilobed nucleus, generally having 3-5 lobes. The cytoplasmic granules are very fine. They stain lightly and therefore the cytoplasm usually appears pale.

b. Eosinophils- These cells generally possess a bilobed nucleus. Large specific granules are present in cytoplasm which take eosin stain and appear red to pink.

c. basophils - In basophils, the nucleus is bilobed or may have an S-shaped appearance. Large and deeply staining granules are found in their cytoplasm. These granules take basic stain and appear blue to purple. Sometimes the granules become so prominent that the nucleus is not clearly seen.

### 2. Agranulocytes

Agranulocytes are leukocytes in which prominent specific granules are not seen under routine staining. However, it does not mean that their cytoplasm is completely without granules. Small azurophilic granules (lysosomal granules) may be present.

The nucleus is generally non-segmented. Lymphocytes and monocytes are included under this group.

a. Lymphocytes have a round or spherical nucleus. In small lymphocytes, the nucleus occupies most part of the cell. Only a thin rim of cytoplasm is seen around it.

b. Monocytes- These are comparatively large agranulocytes with abundant gray-blue cytoplasm. The nucleus is not divided into lobes. It is generally indented, kidney-shaped or C-shaped.

### B. Classification Based on Cell Lineage

Leukocytes can also be classified according to the lineage from which they are developed during hematopoiesis. In this classification, the cells are mainly placed under myeloid lineage and lymphoid lineage.

### 1. Myeloid Lineage

The myeloid lineage produces neutrophils, eosinophils, basophils and monocytes. Thus, all three granulocytes belong to this lineage.

Monocyte is also a myeloid cell. Morphologically it is placed under agranulocytes, which is different from its developmental classification.

Myeloid lineage → Neutrophils + Eosinophils + Basophils + Monocytes

### 2. Lymphoid Lineage

The lymphoid lineage gives rise to lymphocytes. The major cells formed through this pathway are B lymphocytes, T lymphocytes and natural killer (NK) cells.

These cells have the same lymphoid origin, but they undergo different later development.

Lymphoid lineage → B lymphocytes + T lymphocytes + NK cells

## Five Major Types of White Blood Cells and Their Functions

White blood cells are of five major types, neutrophils, eosinophils, basophils, lymphocytes and monocytes. These cells are different in their nucleus, cytoplasmic granules, proportion in blood and immune functions. Neutrophils are present in highest number. Basophils are very few. Some of these cells mainly take part in innate defense, while lymphocytes include the major cells involved in adaptive immunity.

![Neutrophil, eosinophil, basophil, lymphocyte, and monocyte compared by nuclear shape, cytoplasmic granules, relative size, and major immune function.](https://biologynotesonline.com/wp-content/uploads/2024/04/Five-Types-of-White-Blood-Cells-and-Their-Morphology-1024x576.webp)Neutrophil, eosinophil, basophil, lymphocyte, and monocyte compared by nuclear shape, cytoplasmic granules, relative size, and major immune function.

Cell typeRecognizable morphologyApproximate proportionPrincipal functionImmune relationNeutrophilsMultilobed nucleus (3-5 lobes), fine pale granules50-70%Phagocytosis and killing of microorganisms, mainly bacteriaInnateEosinophilsBilobed nucleus, large red-pink or orange granules1-4%Defense against helminths and allergic inflammationMainly innateBasophilsBilobed or S-shaped nucleus, large blue-purple granules<1%Release of histamine and other inflammatory mediatorsMainly innate, IgE-associatedLymphocytesRound dense nucleus, small amount of cytoplasm in small cells20-40%Antibody response, cellular immunity and NK-cell killingAdaptive in B and T cells, innate in NK cellsMonocytesLarge cell, kidney-shaped or indented nucleus, abundant gray-blue cytoplasm2-8%Phagocytosis and antigen presentationInnate and also links with adaptive response

### 1. Neutrophils

[Neutrophils](https://biologynotesonline.com/neutrophil-definition-structure-functions/) are the most abundant WBCs in normal peripheral blood. These are granulocytes having a nucleus divided into usually 3-5 lobes. Fine granules are present in the cytoplasm, but they do not take strong acidic or basic stain. Thus, the cytoplasm appears comparatively pale.

The major function of neutrophils is phagocytosis. They rapidly move towards the area of infection and tissue injury, especially during acute inflammation. Bacteria and cellular materials are engulfed by the cell. In this process, the engulfed material is brought together with intracellular granules and destroyed by granular enzymes and reactive oxygen products.

Neutrophils are therefore mainly cells of innate immunity. Their action is rapid and they form an early cellular response against many bacterial infections.

![Neutrophil engulfing a bacterium into a phagosome followed by granule fusion and destruction using granule enzymes and reactive oxygen products.](https://biologynotesonline.com/wp-content/uploads/2024/04/Neutrophil-Phagocytosis-and-Microbial-Killing-1024x409.webp)Neutrophil engulfing a bacterium into a phagosome followed by granule fusion and destruction using granule enzymes and reactive oxygen products.

### 2. Eosinophils

Eosinophils are granulocytes with a characteristic bilobed nucleus. The cytoplasm contains large specific granules. These granules take eosin strongly and appear red, pink to orange in stained blood smear.

They are mainly involved in defense against multicellular parasites, particularly helminths. The granules contain different proteins which can damage the parasite. Eosinophils are also associated with allergic inflammatory reactions.

Their number is small in normal blood, around 1-4% of total leukocytes. During parasitic infections and several allergic conditions, eosinophil number may increase.

### 3. Basophils

[Basophils](https://biologynotesonline.com/basophil-definition-mechanism-structure-functions/) are the least common WBCs in peripheral blood, generally forming less than 1% of total leukocytes. The nucleus is bilobed or S-shaped. But it is not always clearly visible.

This is because the cytoplasm contains many large, dark staining granules. These granules take basic stain strongly and appear blue to deep purple. Sometimes they almost cover the nucleus.

The major function of basophils is associated with allergic and inflammatory reactions. High-affinity receptors for IgE are present on their surface. When suitable antigen interacts with cell-bound IgE, degranulation can take place. Histamine and other inflammatory substances are then released from the granules.

### 4. Lymphocytes

Lymphocytes are agranular leukocytes. A round and deeply staining nucleus is their main recognizable feature. In small lymphocyte, the nucleus occupies most of the cell and only a narrow rim of cytoplasm can be seen. Larger forms have more cytoplasm.

There are three major lymphocyte groups, B lymphocytes, T lymphocytes and natural killer (NK) cells.

B lymphocytes are involved in antibody-mediated immune response. After proper activation and differentiation, antibody-producing cells are formed from the B-cell lineage. T lymphocytes, on the other hand, are involved in cellular immune responses. Different T-cell groups perform different functions, including regulation of immune responses and killing of infected or abnormal cells.

NK cells have a different role. They can kill certain infected and abnormal cells without the antigen-specific recognition used by B and T lymphocytes. B and T cells are major cells of adaptive immunity, whereas NK cells are included in innate immunity.

### 5. Monocytes

[Monocytes](https://biologynotesonline.com/monocytes/) are large agranular leukocytes having abundant gray-blue cytoplasm. Their nucleus is generally indented, kidney-shaped or C-shaped. They form about 2-8% of circulating WBCs.

Monocytes remain in blood for a period and can then move into tissues. Here, many of these cells develop into macrophages or related mononuclear phagocytic cells.

Their major function is phagocytosis. Microorganisms, damaged cells and cellular debris can be engulfed and broken down by these cells. Monocyte-derived cells can also process antigen and present antigenic material to lymphocytes. Their activity is mainly a part of innate defense, with an important role in connecting innate and adaptive immune responses.

## How Do White Blood Cells Reach Sites of Infection or Injury?

Circulating leukocytes respond to inflammatory signals by adhering to vascular endothelium and migrating from blood into affected tissues. This movement takes place in different steps, mainly in the post-capillary venules. The complete movement from blood to tissue is called leukocyte recruitment or extravasation. Passage of WBC through the endothelial wall is referred to as diapedesis (transmigration).

![Leukocyte recruitment from blood to inflamed tissue through rolling, chemokine activation, firm adhesion, crawling, diapedesis, and chemotaxis.](https://biologynotesonline.com/wp-content/uploads/2024/04/Leukocyte-Extravasation-and-Diapedesis-During-Inflammation-1024x409.webp)Leukocyte recruitment from blood to inflamed tissue through rolling, chemokine activation, firm adhesion, crawling, diapedesis, and chemotaxis.

The following are the steps involved-

### 1. Inflammatory Signals and Activation of Endothelium

During infection or tissue injury, microbial products and damaged tissue components activate the local immune cells. Different cytokines and chemokines are released. These act on the nearby vascular endothelium.

The endothelial cells now become activated. P-selectin and E-selectin are increased on their surface, while ICAM-1 and VCAM-1 are involved in the later steps of adhesion. Chemokines are also displayed on the endothelial surface.

### 2. Margination and Capture

Under normal condition, leukocytes remain moving with the blood. During inflammation, these cells come towards the margin of blood vessel and become closer to the endothelial surface. This is called margination.

A weak contact is then formed between leukocyte and activated endothelium. This initial attachment is referred to as capture or tethering. The attachment is not firm.

### 3. Rolling of Leukocytes

After the initial attachment, leukocytes start rolling over the endothelial surface. The weak bonds are formed and broken repeatedly. Due to this, the WBC moves slowly along the vessel wall.

Selectins play the major role during this process. E-selectin and P-selectin of endothelial cells bind with suitable ligands present on leukocytes (such as PSGL-1). The leukocyte therefore slows down but does not stop completely.

### 4. Activation of Leukocyte

In this step, chemokines present on endothelial surface bind with their receptors on the rolling leukocyte. Intracellular signalling is now initiated in the cell.

The leukocyte integrins are changed from low-affinity condition to an active high-affinity form. The weak rolling contact can now become a strong attachment.

### 5. Firm Adhesion

The activated leukocyte binds firmly with the vascular endothelium. Integrins present on leukocytes take part in this step.

LFA-1 and Mac-1 can bind with ICAM-1, while VLA-4 interacts with VCAM-1. The rolling stops. Leukocyte now remains attached to the endothelial surface and becomes flattened over it.

### 6. Crawling on Endothelium

After firm adhesion, the cell may move along the inner surface of blood vessel before crossing it. This is known as intravascular crawling.

Integrins maintain the attachment during this movement. The leukocyte searches for a suitable endothelial junction or another site through which it can pass out.

### 7. Diapedesis or Transmigration

In this step, leukocyte passes across the vascular endothelium and leaves the blood vessel. This process is referred to as diapedesis (transendothelial migration).

Most leukocytes pass between two adjacent endothelial cells (paracellular route). Some cells can also move directly through the endothelial cell body (transcellular route). PECAM-1 (CD31), JAMs and CD99 are involved during passage through endothelial junctions.

The cell then crosses the underlying basement membrane. It is now present in the tissue.

### 8. Chemotaxis

After coming out from the blood vessel, leukocytes move towards the exact site of infection or tissue damage. This directed movement according to the concentration of chemical substances is called chemotaxis.

Different substances act as chemoattractants, including chemokines (such as CXCL8/IL-8), complement component C5a, leukotriene B4 (LTB4) and bacterial formyl peptides. Leukocytes detect these substances by surface receptors and move towards their higher concentration. Neutrophils show this type of movement strongly during acute inflammation.

## White Blood Cells in Innate and Adaptive Immunity

White blood cells work in a coordinated manner during innate and adaptive immunity. Innate leukocytes respond first to infection and tissue damage, while lymphocytes are activated from the antigens and signals produced during this early response. The adaptive response again acts with innate cells through antibodies, cytokines and direct cell interactions.

![Dendritic cells, neutrophils, macrophages, B cells, T cells, NK cells, eosinophils, and basophils coordinating innate and adaptive immune responses.](https://biologynotesonline.com/wp-content/uploads/2024/04/Innate-and-Adaptive-White-Blood-Cell-Cooperation-1024x683.webp)Dendritic cells, neutrophils, macrophages, B cells, T cells, NK cells, eosinophils, and basophils coordinating innate and adaptive immune responses.

### Early Innate Immune Response

During infection, tissue macrophages and other sensing cells recognize microbial or damaged-cell components. Inflammatory substances are released. These signals recruit neutrophils and monocytes from blood into the affected tissue.

The recruited cells engulf microorganisms and damaged materials. [Natural killer (NK) cells](https://biologynotesonline.com/natural-killer-nk-cells/) can also act at this early stage against some infected or abnormal cells. Cytokines and chemokines produced during the process bring in and activate further leukocytes.

### Connection Between Innate and Adaptive Immunity

The information from innate response is passed to adaptive immune cells mainly through dendritic cells. These cells take up antigens in tissues and, after activation, move towards the draining lymphoid organs.

In this step, processed antigen is presented to naïve T lymphocytes together with costimulatory signals. T cells having suitable antigen receptor become activated and multiply.

### Coordination by T Lymphocytes

[Helper T lymphocytes](https://biologynotesonline.com/t-cell-t-lymphocyte/) release cytokines which act on other leukocytes. Some of these signals increase the antimicrobial activity of macrophages. Other signals help in activation and differentiation of B lymphocytes.

Cytotoxic T cells, on the other hand, act directly on infected cells carrying the corresponding antigen. The infected cells are killed.

### Antibody and Phagocytic Cell Cooperation

Activated [B lymphocytes](https://biologynotesonline.com/b-cell-or-b-lymphocytes-definition-function-types-applications/) develop into antibody-producing cells. Antibodies bind specifically with their corresponding antigen, but removal of the antigen commonly involves other immune components.

For example, antibody-coated microorganisms are recognized by Fc receptors present on phagocytic leukocytes. The microorganisms are then engulfed and destroyed. Antibodies can also activate complement, which helps in opsonization and recruitment of inflammatory cells.

### Antibodies and NK Cells

Antibody-coated target cells can also be attacked by NK cells. NK cells possess FcγRIII (CD16) which binds to the Fc region of IgG present on the target cell.

After this binding, cytotoxic granules are released from the NK cell and the target cell is killed. This process is referred to as antibody-dependent cellular cytotoxicity (ADCC).

### Cooperation During Parasite and Allergic Responses

During type 2 immune responses, eosinophils and basophils work with lymphocyte-mediated mechanisms. T helper 2 (TH2) cells provide signals which favor IgE production by B cells.

IgE can bind with high-affinity receptors present on basophils and activated eosinophils. When the corresponding antigen is encountered, these cells become activated and granular as well as inflammatory substances are released.

## What Does a WBC Count and Differential Measure?

A total white blood cell (WBC) count measures the total concentration of leukocytes present in blood, while the WBC differential separates these leukocytes according to their type. Both are commonly reported as a part of complete blood count (CBC) with differential. The total count gives one combined WBC value. Differential, on the other hand, shows neutrophils, lymphocytes, monocytes, eosinophils and basophils separately.

### Total White Blood Cell Count

The total WBC count is the number of all circulating leukocytes present in a particular volume of blood. Neutrophils, lymphocytes, monocytes, eosinophils and basophils are counted together in this value.

For adults, a representative reference interval is approximately 4,500-11,000 cells/µL (4.5-11.0 × 10⁹/L). This is an example range. Different laboratories can use slightly different reference intervals and measurement systems.

The test only tells the overall number of WBCs. It does not show which type of leukocyte is responsible for a change in the total count. For this, differential count is used.

### Differential White Blood Cell Count

The differential WBC count measures the amount or percentage of each major type of leukocyte separately. The following are measured- neutrophils, lymphocytes, monocytes, eosinophils and basophils. Abnormal or immature cells may also be detected during differential examination.

Note- The values below are representative adult percentages only. Reference intervals can differ according to laboratory, age and physiological or clinical condition. The reference interval printed by the testing laboratory is used for interpretation.

WBC typeApproximate differential percentageWhat the differential measuresNeutrophils40-60%Proportion of neutrophils among total circulating WBCsLymphocytes20-40%Proportion of lymphocytesMonocytes2-8%Proportion of monocytesEosinophils1-4%Proportion of eosinophilsBasophils0.5-1%Proportion of basophils

The differential may be reported as relative percentage or as an absolute count. Percentage shows how much of the total WBC population belongs to a particular leukocyte type. Absolute count gives the actual number of that particular cell type in a volume of blood.

For example, if neutrophils form 60% of a total WBC count of 8,000 cells/µL, the absolute neutrophil count is obtained from-

8,000 × 60/100 = 4,800 neutrophils/µL

Absolute counts can be more informative than percentage alone. A percentage may change because another leukocyte population has increased or decreased, even when the actual number of that particular cell has not changed to the same extent.

A WBC count or differential value is therefore read with the laboratory reference interval and other blood findings. An isolated high or low value by itself does not establish a particular diagnosis.

## Leukocytosis and Leukopenia: Interpreting Changes in WBC Numbers

Changes in white blood cell number are mainly described as leukocytosis and leukopenia. Leukocytosis refers to an increased WBC count, whereas leukopenia is a decrease below the normal reference range. These changes can occur by different mechanisms. The type of leukocyte that increases or decreases is therefore also checked with the total count.

### Leukocytosis — Increased White Blood Cell Count

Leukocytosis is an increase in the total number of circulating white blood cells. It can occur when more leukocytes are produced or released from bone marrow. Movement of marginated WBCs into circulating blood (demargination) can also increase the count.

Some of the important causes are-

Infection- WBC production and release can increase during infection. Neutrophils are commonly increased during many bacterial infections.

Inflammation- Acute as well as chronic inflammatory conditions may produce leukocytosis.

Physiological stress- Vigorous exercise, physical stress and emotional stress can temporarily increase circulating WBCs. Stress hormones are involved in this process.

Medications- Some drugs can increase leukocyte number. Corticosteroids and certain growth factors are common examples.

Hematologic disease- Excessive or abnormal production of leukocytes can also take place in disorders of blood-forming cells.

Thus, an increased total WBC count does not by itself show the exact cause. The differential count, duration of the change and other blood findings are also considered.

### Leukopenia — Decreased White Blood Cell Count

Leukopenia is a decreased total number of circulating WBCs. Reduced production in bone marrow is one major mechanism. Increased destruction, consumption or removal of leukocytes from circulation can also produce a low count.

Bone marrow production may be suppressed by chemotherapy, radiation and some medications. Selected infections can also reduce WBC numbers. In some cases, immune mechanisms destroy circulating leukocytes or their precursors.

The decrease often involves neutrophils, producing neutropenia. Neutropenia can develop from reduced marrow production as well as peripheral destruction or consumption.

### Why the Differential Matters

A total WBC count combines all leukocytes into one value. For this reason, two individuals can have a similar total count but a different change in leukocyte population. The differential shows which cell type is mainly involved.

For example, neutrophilia means an increase in neutrophils. It can accompany infection and inflammation, but it can also occur with physiological stress or some medications. Therefore neutrophilia itself is not a diagnosis.

Eosinophilia, on the other hand, means an increased eosinophil count. It can be associated with allergic responses or parasitic conditions. The biological meaning is therefore different from a rise mainly caused by neutrophils.

The same principle applies when a leukocyte type falls. A low total WBC value caused mainly by reduction of neutrophils has different biological importance than a reduction involving another leukocyte population.

## White Blood Cells Compared with Red Blood Cells and Platelets

White blood cells (WBCs), red blood cells (RBCs) and platelets are the major formed elements present in blood, but their structure and functions are different. WBCs are complete nucleated cells involved mainly in body defense. Mature RBCs are anucleate cells specialized for transport of respiratory gases, while platelets are small cytoplasmic fragments mainly used during hemostasis.

FeatureWhite blood cells (Leukocytes)Red blood cells (Erythrocytes)PlateletsBasic structureComplete cells having plasma membrane, cytoplasm, nucleus and cellular organellesMature cells lack nucleus and most cellular organellesSmall membrane-covered cytoplasmic fragments released from megakaryocytesNucleusPresent. Shape differs among different WBCsAbsent in mature RBCsAbsentGeneral appearanceVariable in size and shape. Different WBCs have different nuclear forms and cytoplasmic granulesBiconcave disc-shaped cells, about 7–8 µm in diameterVery small fragments, generally about 2–4 µmHemoglobinAbsentPresent in large amountAbsentMajor functionImmune defense against microorganisms, damaged cells and other harmful agentsMainly transport of oxygen and part of carbon dioxideHemostasis, blood clot formation and support of tissue repairMovement from blood vesselsMany WBCs can leave blood vessels and move into tissues by diapedesisNormally remain within the vascular systemCirculate in blood and become activated at sites of vascular injuryNumber in bloodMuch less numerous than RBCsMost numerous formed element of bloodMore numerous than WBCs but much less than RBCsMain biological roleDefense and immune responseRespiratory-gas transportPrevention of blood loss after vessel damage

## Functions of WBC

White blood cells (WBCs) perform the defensive and immune functions of the body against microorganisms, damaged cells and other foreign materials. Different WBCs have different functions. Some engulf the microorganisms, some form antibodies, while other cells directly kill infected or abnormal cells.

Some of the important functions of WBCs are-

- Defense against infection- Protection against bacteria, viruses and other infectious agents is one of the major functions of WBCs. Depending upon the infection, different leukocytes are involved.

- Phagocytosis- Microorganisms, dead cells and cellular debris are engulfed mainly by neutrophils and macrophages. This process is referred to as phagocytosis.

- Inflammatory response- During inflammation, leukocytes are brought from blood into the affected tissues. Neutrophils and monocytes are recruited, while inflammatory substances such as histamine are released by basophils.

- Adaptive immunity- B and T lymphocytes are involved in adaptive immune response. Antibody-mediated immunity is mainly carried out by B cells. T cells, on the other hand, perform different cellular immune functions.

- Antibody production- From activated B lymphocytes, plasma cells are formed which produce antibodies against their specific antigens.

- Killing of infected and abnormal cells- Direct killing is also performed by some WBCs. Cytotoxic T cells act against infected cells carrying the corresponding antigen, whereas natural killer (NK) cells kill certain infected and abnormal cells.

- Defense against parasites- Eosinophils have a major role against multicellular parasites (especially helminths), with their granular proteins damaging the parasite.

- Allergic reactions- Basophils and eosinophils also take part in allergic responses. During basophil degranulation, histamine and other inflammatory substances are released.

- Antigen presentation- Antigenic materials are processed by macrophages and presented to T lymphocytes, helping in activation of adaptive immune response.

- Regulation of immune cells- Different leukocytes also regulate one another through cytokines. Helper T cells and macrophages have an important role in this process.

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