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Blood, Lymphatic, and Immune Systems

Introduction

Homeostasis, or a "steady state," is a continual balancing act of the body

systems to provide an internal environment that is comparable with life. The

two liquid tissues of the body, the blood and lymph have separate but

interrelated functions in maintaining this balance. They combine with a third

system, the immune, to protect the body against pathogens that could threaten

the organism's viability. The blood is responsible for the following:

Transportation of gases (oxygen O2) and carbon dioxide (CO2), chemical

substances (hormones, nutrients, salts), and cells that defend the body.

Regulation of the body's fluid and electrolyte balance, acid-base balance,

and body temperature.

Protection of the body from infection.

Protection of the body from loss of blood by the action of clotting.

The lymph system is responsible for the following:

Cleansing the cellular environment

Returning proteins and tissue fluids to the blood (drainage)

Providing a pathway for the absorption of fats and fat-soluble vitamins

into the bloodstream.

Defending the body against disease.

The immune system is responsible for the following:

Defending the body against disease via the immune response

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The hematic and lymphatic systems flow through separate yet interconnected

and interdependent channels. Both are systems composed of vessels and the

liquids that flow through them. The immune system, a very complex set of

levels of protection for the body, includes blood and lymph cells.

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The above graphics show the relationship of the lymphatic vessels to the

circulatory system. Note the close relationship between the distribution of the

lymphatic vessels and the venous blood vessels.

Filtration in capillaries creates tissue fluid, most of which returns almost

immediately to the blood in the capillaries by osmosis. Some tissue fluid,

however, remains in interstitial spaces and must be returned to the blood by way

of the lymphatic vessels. Without this return, blood volume and blood pressure

would very soon decrease. Tissue fluid is drained by the lymphatic capillaries

and transported by a series of larger lymphatic vessels into the circulation.

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Hematic System (Blood)

What is Blood?

Blood is the fluid that circulates in the peripheral vascular system of humans.

Blood is a tissue. It is classified as a special type of connective tissue. Blood is

considered a connective tissue for two basic reasons: (1) embryologically, it has

the same origin (mesoderm) as do the other connective tissue types and (2)

blood connects the body systems together bringing the needed oxygen,

nutrients, hormones and other signaling molecules, and removing wastes.

Characteristics of blood

Blood has distinctive physical characteristics:

Amount: an adult person has 4 to 6 liters of blood, depending on his or her size

(around 7-8% of weight). Of the total blood volume in the human body, 38% to

48% is composed of the various blood cells, also called "formed elements". The

remaining 52% to 62% of the blood volume is plasma, the liquid portion of

blood.

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Color: blood color varies; arterial blood is bright red because it contains high

levels of oxygen. Venous blood have given up much of its oxygen in tissues,

and has a darker, dull red color. This may be important in the assessment of the

source of bleeding. If blood is bright red, it is probably from a severed artery,

and the dark red blood is probably venous blood.

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pH: the normal pH range of blood is 7.35 to 7.45, which is slightly alkaline.

Venous blood normally has a lower pH than does arterial blood because of the

presence of more carbon dioxide.

Viscosity: this means thickness or resistance to flow. Blood is about three to

five times thicker than water. Viscosity is increased by the presence of blood

cells and the plasma proteins, and this thickness contributes to normal blood

pressure.

Plasma

Plasma is the liquid part of blood and is approximately 90% water. The solvent

ability of water enables the plasma to transport many types of substances (water

is often regarded as a universal solvent).

Function of plasma

Nutrients absorbed in the digestive tract are circulated to all body tissues, and

waste products of the tissues circulate through the kidneys and are excreted in

urine. Hormones produced by endocrine glands are carried in the plasma to their

target organs, and antibodies are also transported in plasma.

Most of the carbon dioxide produced by cells is carried in the plasma in the

form of bicarbonate ions (HCO3-). When the blood reaches the lungs, the CO2 is

re-formed, diffuses into alveoli and is exhaled.

Plasma also carries blood heat. Blood is warmed by flowing through active

organs such as the liver and muscles. This heat is distributed to cooler parts of

the body as blood continues to circulate.

Plasma is composed of the following:

1. Water, or H2O (90%)

2. Inorganic substances (calcium, potassium, sodium)

3. Organic substances (glucose, amino acids, fats, cholesterol, hormones)

4. Waste products (urea, uric acid, ammonia, creatinine)

5. Plasma proteins (serum albumin, serum globulin, and two clotting

proteins: fibrinogen and prothrombin)

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Plasma proteins

Albumin is the most abundant plasma protein. It is synthesized by the liver.

Albumin contributes to the colloid osmotic pressure of blood, which pulls tissue

fluid into capillaries. This is important to maintain normal blood volume and

blood pressure. Albumin also serves as a transport protein, particularly for fatty

acids and several hydrophobic steroid hormones.

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Other plasma proteins are called globulins. Alpha and beta globulins are

synthesized by the liver and act as carriers for molecules such as fats. The

gamma globulins are antibodies produced by lymphocytes. Antibodies initiate

the destruction of pathogens and provide us with immunity.

The clotting factors prothrombin, fibrinogen, and others are also synthesized

by the liver and circulate until activated to form a clot in a ruptured or damaged

blood vessel. Fibrinogen is soluble in plasma. It is converted into fibrin as part

of the clotting cascade. Fibrin fibers form the frame for the blood clot.

Type Site of synthesis Percentage Function

Albumin Liver 60% colloid osmotic

pressure of blood

Alpha globulins Liver 35% carriers for

molecules such as

fats Beta globulins

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Gamma globulins Lymphocytes Produce antibodies

Fibrinogen Liver 4 % Blood Coagulation

Prothrombin Liver 1 %

Fibrin Sealant

Fibrin sealant is a form of surgical glue. The fibrin sealants are comprised of

purified, virus-inactivated human fibrinogen, human thrombin, and sometimes

added components, such as virus-inactivated human factor XIII and bovine

aprotinin. Fibrin sealants are the most effective tissue adhesives currently

available, and they are biocompatible and biodegradable. The drawing below

shows the use of fibrin sealant in the treatment of anal fistula (Fistula-in-ano).

Serum

Serum is plasma minus the clotting proteins. Serology is the branch of

laboratory medicine that studies blood serum for evidence of infection by

evaluating antigen-antibody reactions in vitro.

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Blood Cells (Formed elements)

The solid portion of blood is composed of three different types of cells:

Erythrocytes - also called red blood cells (RBCs).

Leukocytes - also called white blood cells (WBCs).

o Granulocytes / Polymorphonuclear leukocytes (Neutrophils,

Eosinophils, Basophils)

o Mononuclear leukocytes / Agranulocytes (Lymphocytes,

Monocytes)

Thrombocytes - also called platelets.

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Erythrocytes (Red Blood Cells)

The erythrocytes (which are normally present in the millions) have the

important function of transport O2 and CO2 throughout the body. The vehicle

for this transportation is a protein-iron pigment called hemoglobin.

Traditionally, an erythrocyte was viewed as "an outer plasma membrane

enclosing hemoglobin and a limited number of enzymes necessary for

maintenance of plasma membrane integrity and gas transport function".

RBCs normally have a biconcave, disc-like shape, which means their centers

are thinner than their edges. Red blood cells are the only human cells without

nuclei. Typically, an erythrocyte's diameter is 7-8 um. The biconcave shape

provides around 30% more surface area, compared to spherical form. The larger

surface area greatly enhances gas exchange. Additionally, this shape allows

RBCs to "squeeze" themselves through the smallest capillaries (around 3 um in

diameter).

Abnormal RBCs can be named by their morphology. Those that are shaped

differently often have difficulty in carrying out their function. For example,

sickle cell anemia is a hereditary condition characterized by erythrocytes

(RBCs) that are abnormally shaped. They resemble a crescent or sickle. An

abnormal hemoglobin found inside these erythrocytes causes sickle-cell anemia

in a number of Africans and African-Americans. (Reflection: Is sickle cell

always bad?)

Pikilocytes

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1 = Irreversibly sickled red cell; 2 = Nucleated red cell (orthochromatic erythroblast) Another example is spherocytosis, as RBCs assume the shape of spherocytes.

The primary lesion in spherocytosis is loss of membrane surface area, leading to

reduced deformability due to defects in certain membrane proteins. Abnormal

spherocytes are trapped and destroyed in the spleen and this is the main cause of

haemolysis in this disorder.

Rouleaux appeareance

RBCs appear adherent together like a coin

It occurs in slow circulation

It is reversible

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EM

RBCs are non nucleated cells devoid of cell organelles

It contains haemoglobin only

Lacking mitochondria, erythrocytes rely on anaerobic glycolysis for their

minimal energy needs.

Lacking nuclei, they cannot replace defective proteins

Osmotic fragility of RBCs

Hypertonic solution …………………….crenation

Hypotonic solution ……………………….Haemolysis

Colour

The RBCs is acidophilic , this called normochromic

Count

In Male 4.7 to 6.1 million /mm³

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In Females 4.2 – 5.4 million /mm³

The count is variable with age it increase in newborns and decrease gradually

Abnormal count

Decrease Number (Anaemia )

Deficiency anaemia

Deficient any important factor for RBCs formation like iron ,Vit.B12 ,folic acid

or protein

pernicious anaemia : sever deficiency of Vit. B12

Aplastic anaemia: damage of bone marrow due to radiation or chemotherapy

Heamolytic anaemia : Destruction of RBCs due to

Abnormal shape (congenital spherocytosis)

Abnormal content (Sickle shape or favism)

Increase Number (Polycythaemia)

1 - False due to hemoconcentration (dehydration or burn)

2 - True

Physiological : High attitude ,Exercise or newborn

Pathological : Heart and lung disease

Erythrocyte Membrane Structure

The erythrocyte membrane consists of two domains, a lipid bilayer and the

cytoskeleton. The lipid domain is similar structurally to that found in most

mammalian cells. The cytoskeleton differs from what is considered cytoskeleton

in other cells because it does not contain the structural protein tubulin and is not

involved in cell motility or phagocytosis.

Lipid Domain

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The lipid domain is composed of nearly equal parts of lipid and protein. The

main lipids are cholesterol and phospholipids. Lipid domain fluidity is

determined by the molar ratio of cholesterol to phospholipid, degree of

unsaturation of phospholipid acyl chains, and phosphatidylcholine to

sphingomyelin ratio. Phosphatidylcholine forms highly fluid lipid regions, while

sphingomyelin induces rigidity.

Cytoskeleton

The erythrocyte cytoskeleton consists of several proteins that form a

filamentous network under the lipid bilayer. The network is composed of

spectrin, ankyrin, actin, and protein 4.1. Cytoskeletal proteins interact with

integral proteins and lipids of the bilayer to maintain membrane integrity. The

cytoskeleton has an important role in erythrocyte shape, flexibility, and lipid

organization.

Hemoglobin

Hemoglobin consists of the protein globin and heme pigment. Globin Consists

of two and two subunits. Each subunit binds to a heme group. Each heme

group bears an atom of iron, which binds reversibly with one molecule of

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oxygen. So, each hemoglobin molecule carries four molecules of oxygen. On

the other hand, carbon monoxide competes with oxygen for heme binding with

a much higher affinity. (Smoking and Hct)

Erythropoiesis

It is the pathway through which an erythrocyte matures from a hemocytoblast

(hematopoietic stem cell) into a full-blown erythrocyte. The rate of production

is very rapid (estimated at several million new RBCs per second) and a major

regulating factor is oxygen. If the body is in a state of hypoxia or lack of

oxygen, the kidneys produce a hormone called erythropoietin, which stimulates

the red bone marrow to increase the rate of RBC production. This will occur

following hemorrhage, or if a person stays for a time at a higher altitude. As a

result of the action of erythropoietin, more RBCs will be available to carry

oxygen and correct the hypoxic state.

Erythrocyte differentiation takes place in several stages over a period of 1 week.

They all take place within the bone marrow (in the adult). Within the bone

marrow, erythroid progenitors are found in the form of "islands", called

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erythroid colonies. An erythroid colony is composed of erythroblasts

surrounding a central macrophage. The more immature precursors are present

close to the macrophage and maturing forms are towards the periphery. The

macrophage has dendritic processes which extend between erythroid

progenitors, support them, and supply iron for hemoglobin synthesis.

The figure below shows Micrographs of erythroblastic islands. (A)

Transmission electron micrograph of an erythroblastic island isolated from bone

marrow. Note the extensive cell-cell contact. (B) Scanning electron micrograph

of an isolated erythroblastic island. The inset shows an optical microscopic

image of the same structure. Note the presence of an enucleating erythroblast

(➔) and a multilobulated reticulocyte (➤). (C) Confocal immunofluorescence

image of an island reconstituted from freshly harvested mouse bone marrow

cells stained with erythroid-specific marker (red), macrophage marker (green)

and DNAprobe (blue). Central macrophage is indicated by an arrow and a

multilobulated reticulocyte by an arrowhead.

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With each stage of erythrocyte differentiation, cell size and nuclear size become

smaller and chromatin clumping increases. Color of cytoplasm changes from

basophilic to orange-red due to increased accumulation of hemoglobin.

The earliest morphologically identifiable erythroid cell in the bone marrow is

the proerythroblast (pronormoblast), a large (15-20 um) cell with a fine,

uniform chromatin pattern, one or more nucleoli, and dark blue cytoplasm. The

next cell in the maturation process is the basophilic (early) normoblast. This

cell is smaller in size (12-16 um) and has a coarser nuclear chromatin with

barely visible nucleoli. The cytoplasm is deeply basophilic.

The more differentiated erythroid cell is the polychromatic (intermediate)

normoblast (size 12-15 um). The nuclear size is smaller and the chromatin

becomes clumped. Polychromasia of cytoplasm results from admixture of blue

RNA and pink hemoglobin. This is the last erythroid precursor capable of

mitotic division. The orthochromatic (late) normoblast is 8 to 12 um in size.

The nucleus is small, dense and pyknotic and commonly eccentrically-located.

The cytoplasm stains mostly pink due to hemoglobinisation. It is called

orthochromatic because cytoplasmic staining is largely similar to that of

erythrocytes.

When the late normoblast ejects its nucleus, it becomes a reticulocyte, so called

because of a reticular (mesh-like) network of ribosomal RNA visible under a

microscope with certain stains such as new methylene blue and Romanowsky

stains. The reticulocyte is released into the blood stream, where it then matures

1-2 days later into an erythrocyte. Reticulocytes make up 1% of RBCs in

peripheral blood. Large numbers of reticulocytes or normoblasts in the

circulating blood mean that the number of mature RBCs is not sufficient to

carry the oxygen needed by the body. Such situations include hemorrhage, or

when mature RBCs have been destroyed, as in Rh disease of the newborn, and

malaria.

In summary, erythrpoeisis stages are as follows:

1. Hemocytoblast, or hematopoietic stem cell, which is a multipotent stem

cell.

2. Common myeloid progenitor, a multipotent stem cell.

3. Megakaryocyte and erythroid precursor, a committed precursor cell

4. Erythroid progenitor cell, a unipotent stem cell or lineage committed cell

5. Pronormoblast, or proerythroblast

6. Basophilic or early normoblast, also called an erythroblast.

7. Polychromatophilic or intermediate normoblast

8. Orthochromatic or late normoblast

9. Reticulocyte

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The maturation of red blood cells requires many nutrients. Protein and iron are

necessary for the synthesis of hemoglobin and become part of hemoglobin

molecules. The vitamins folic acid and B12 are required for DNA synthesis in

the stem cells of the red bone marrow. As these cells undergo mitosis they must

continually produce new sets of chromosomes. Vitamin B12 is also called the

extrinsic factor because its source is external, our food. Parietal cells of the

stomach lining produce the intrinsic factor, a chemical that combines with the

vitamin B12 in food to prevent its digestion and promote its absorption in the

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small intestines. A deficiency of either vitamin B12 or in the intrinsic factor

results in a type of anemia called pernicious anemia.

Life span

Red blood cells live for approximately 120 days. As they reach this age they

become fragile and are removed from circulation by cells of the tissue

macrophage system (formerly called the reticuloendothelial or RE system).

The organs that contain macrophages are the liver, spleen, and red bone

marrow. The old RBCs are phagocytized and digested by macrophages, and the

iron they contained is put into the blood to be returned to the red bone marrow

to be used for the synthesis of new hemoglobin. If not needed immediately for

this purpose, excess iron is stored in the liver. The iron of RBCs is actually

recycled over and over again.

Another part of the hemoglobin molecule is the heme portion, which cannot be

recycled and is a waste product. The heme is converted to bilirubin by

macrophages. The liver removes bilirubin from circulation and excretes it into

bile; bilirubin is called a bile pigment. Bile is secreted by the liver into the

duodenum and passes through the small intestine and colon, so bilirubin is

eliminated in feces, and gives feces their characteristic brown color. If bilirubin

is not excreted properly, perhaps because of liver disease such as hepatitis, it

remains in the blood. This may cause jaundice, a condition in which the whites

of the eyes appear yellow. This yellow color may also be seen in the skin of

light-skinned people.

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Blood Groups

Human blood is divided into four major different types: A, B, Ab, and O. The

differences are due to antigens present on the surface of the blood cells.

Antigens are substances that produce an immune reaction by their nature of

being perceived as foreign to the body. In response, the body produces

substances called antibodies that nullify or neutralize the antigens. In blood,

these antigens are called agglutinogens because their presence can cause the

blood to clot.

The antibody is termed an agglutinin. For example, type A blood has A

antigen, type B has B antigen, type AB has both A and B antigens, and type O

has neither A nor B antigens. Following the logic of each of these antigen-

antibody reactions, an individual with type AB blood is a universal recipient,

and an individual with type O blood is a universal donor.

The Rh factor is another antigen (often called D) that may be present on RBCs.

People whose RBCs have the Rh antigen are Rh positive, those without the

antigen are Rh negative. Rh negative people do not have natural antibodies to

the Rh antigen, and for them this antigen is foreign. If an Rh negative person

receives Rh positive blood by mistake, antibodies will be formed just as they

would be to bacteria or viruses.

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The Rh factor is important in pregnancy because a mismatch between the fetus

and the mother can cause erythroblastosis fetalis, or hemolytic disease of the

newborn. In this disorder, a mother with a negative Rh factor will develop

antibodies to an RH + fetus during the first pregnancy. If another pregnancy

occurs with an Rh + fetus, the antibodies will destroy the fetal blood cells.

Leukocytes (White Blood Cells)

White blood cells (WBCs) are also called leukocytes. There are five kinds of

WBCs; all are larger than RBCs and have nuclei when mature. The nucleus may

be in one piece or appear as several lobes. Special staining for microscopic

examination gives each kind of WBC a distinctive appearance.

A normal WBC count (part of a CBC) is 5,000 to 10,000 per mm3. Notice that

this number is quite small compared to a normal RBC count. Many of our

WBCs are not within blood vessels but are carrying out their functions in tissue

fluid.

Classification and sites of production

The five kinds of white blood cells may be classified in two groups: granular

and agranular. The granular leukocytes are produced in the red bone marrow;

these are the neutrophils, eosinophils, and basophils, which have distinctly

colored granules when stained. The agranular leukocytes are lymphocytes and

monocytes, which are produced in the lymphatic tissue of the spleen, lymph

nodes, and thymus, as well as in the red bone marrow. A differential WBC

count (part of a CBC) is the percentage of each kind of leukocyte.

Functions

White blood cells all contribute to the same general function, which is to protect

the body from infectious disease and to provide immunity to certain diseases.

Each kind of leukocyte has a role in this very important aspect of homeostasis.

Neutrophils and monocytes are capable of the phagocytosis of pathogens.

Neutrophils are the more abundant phagocytes, but monocytes are the more

efficient phagocytes, because they differentiate into macrophages, which also

phagocytize dead or damaged tissue at the site of any injury, helping to make

tissue repair possible.

Eosinophils are believed to detoxify foreign proteins. This is especially

important in allergic reactions and parasitic infections. Basophils contain

granules of heparin and histamine. Heparin is an anticoagulant that helps

prevent abnormal clotting within blood vessels. Histamine is released as part of

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the inflammation process, and it makes capillaries more permeable, allowing

tissue fluid, proteins and white blood cells to accumulate in the damaged area.

There are two major kinds of lymphocytes: T cells and B cells. T cells (or T

lymphocytes) recognize foreign antigens, may directly destroy some foreign

antigens, and stop the immune response when the antigen has been destroyed. B

cells (or B lymphocytes) become plasma cells that produce antibodies to

foreign antigens.

Leukocytes function in tissue fluid as well as in the blood. Many WBCs are

capable of self-locomotion (ameboid movement) and are able to squeeze

between the cells of capillary walls and out into tissue spaces. Macrophages

provide a good example of dual location of leukocytes. Some macrophages are

"fixed", that is stationary in organs such as the liver, spleen, and red bone

marrow (part of the tissue macrophage or RE system) and in the lymph nodes.

They phagocytize pathogens that circulate in blood or lymph through these

organs (these are the same macrophages that also phagocytize old RBCs). Other

"wandering" macrophages move about in tissue fluid, especially in the

connective tissue of mucous membranes and below the skin. Pathogens that

gain entry into the body through natural openings or through breaks in the skin

are usually destroyed by the leukocytes in connective tissue before they can

cause serious disease.

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Neutrophils

Stages of Granulopoiesis

The maturation sequence in granulopoiesis is myeloblast, promyelocyte,

myelocyte, metamyelocyte, band cell, and segmented granulocyte. This process

occurs within the bone marrow. G-CSF is a key regulator of neutrophil

production.

The myeloblast is derived from the granulocyte monocyte progenitor cell,

which is in turn derived from the common myeloid progenitor cell. The

myeloblast is the earliest recognizable cell in the granulocytic maturation

process. It is about 15 to 20 um in diameter, with a large round to oval nucleus,

and small amount of basophilic cytoplasm, with no granules. The nucleus

contains 2 to 5 nucleoli and nuclear chromatin is fine and reticular. The next

stage in the maturation is promyelocyte which is slightly larger in size than

myeloblast. Primary or azurophilic granules appear at the premyelocyte stage.

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The nucleus contains nucleoli as in myeloblast stage, but nuclear chromatin

shows slight condensation.

Myelocyte stage is characterized by the appearance of secondary or specific

granules (neutrophilic, eosinphilic, or basophilic). Neutrophilic myelocyte is a

small cell with round to oval eccentrically placed nucleus, more condensation of

chromatin than in premyelocyte stage, and absence of nucleoli. Cytoplasm is

relatively greater in amount than in the premyelocyte stage and contain both

primary and secondary granules. Myelocyte is the last cell capable of mitotic

division. In the neurtophilic metamyelocyte stage, the nucleus becomes

indented and kidney shaped, and the nuclear chromatin becomes moderately

coarse. Cytoplasm contains both primary and secondary granules. The band

(stab) cell stage is characterized by horseshoe shape of the nucleus with

constant diameter throughout and condensed nuclear chromatins. Band cells are

rarely seen in blood film. An increase in band neutrophils typically means that

the bone marrow has been signaled to increase production of leukocytes, also

known as a "left shift". Most often this is due to infection or inflammation in the

body.

With Leishman's stain, the nucleus of the segmented (polymorphonuclear)

neutrophil appears deep purple with 2 to 5 lobes which are joined by thin

filamentous strands. Nuclear chromatin pattern is coarse. The cytoplasm stains

light pink and has small specific granules.

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Neutrophilic granules

The neutrophil granules are of two types: primary or azurophilic granules and

secondary or specific granules. Azurophilic granules contain myeloperoxidase,

lysozyme, acid phosphatase, elastases, collagenases, and acid hydrolases.

Specific granules contain lysozyme, lactoferrin, alkaline phosphatases, and

other substances.

Function of neutrophils

After their formation, neutrophils remain in bone marrow for 5 more days as a

reserve pool. Neutrophils have a life span of only 1 to 2 days in circulation. In

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response to infection and inflammation, neutrophils come to lie closer to

endothelium (margination) and adhere to endothelial surface (sticking). This is

followed by escape of neutrophils from blood vessels to extravascular tissue

(emigration). The escape of neutrophils is guided by chemotactic factors present

in the inflammatory zone. Phagocytosis follows, which involves three steps:

antigen recognition, engulfment, and killing the organism.

Neutrophils are the most abundant type of leukocytes in the blood. Their main

function is in the acute inflammatory response. An increase in the number of

neutrophils is termed neutrophilia, mostly seen in the course of bacterial

infections. Some drugs, such as prednisone, have the same effect as cortisol and

adrenaline (epinephrine), causing marginated neutrophils to enter the blood

stream, causing neutrophilia.

Eosinophils

The eosinophil forms via the same stages as the neutrophil, with the exception

that IL-5 ismost implicated in eosinophil production. Eosinphilic specific

granules first become evident at the myelocyte stage. The size of the eosinophil

is slightly greater than that of a neutrophil. The nucleus is often bilobed and the

cytoplasm contains numerous, large, bright orange-red granules. The granules

contain major basic protein, cationic protein, and peroxidase (which is distinct

from myeloperoxidase). Eosinophilic peroxidase along with iodide and

hydrogen peroxide may be responsible for some defense against helminthic

parasites. Maturation time for eosinophils in bone marrow is 2 to 6 days and

half-life in blood is less than 8 hours. In tissues, they reside in skin, lungs, and

gastrointestinal tract.

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Basophils

Basophils are small, round to oval cells which contain very large, coarse, deep

purple granules. The nucleus has condensed chromatin and is covered by

granules. Basophil granules contain histamine, chondroitin sulfate, heparin,

proteases and peroxidase. Basophils bear surface membrane receptors for IgE.

Upon reaction of antigen with membrane-bound IgE, histamine and other

granular contents are released which play a role in immediate hypersensitivity

reaction. Basophils are also involved in some cutaneous basophil

hypersensitivity reactions.

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Monocytes

The initial cell in development is monoblast, which is indistinguishable from

myeloblast. The next cell is promonocyte which has an oval or clefted nucleus

with fine chromatin pattern and 2 to 5 nucleoli. The monocyte is a large cell

(15-20 um) with irregular shape, oval or clefted (often kidney-shaped) nucleus,

and fine delicate chromatin. Cytoplasm is abundant, blue-grey with ground

glass appearance and often contains fine azurophilic granules and vacuoles.

Monocytes circulate in blood for about 1 day and then enter and settle in tissues

where they differentiate into macrophages. Macrophage phagocytosis is slower

as compared to neutrophils. In some organs, macrophages have distinctive

morphologic and functional characteristics. Examples include Kupffer cells in

the liver, Alveolar macrophages in the lungs, osteoclasts in the bone, and others.

Platelets

Blood platelets (or thrombocytes) are very small non-nucleated,

membrane-bound cell only 2-4 μm in diameter

Platelets originate from megakaryocytes.

Platelets promote blood clotting and help repair minor tears or leaks in

the walls of small blood vessels.

Normal platelet counts range from 150,000 to 400,000/μL (mm3) of

blood.

Life span about 10 days

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Its shape is discoid, with a very lightly stained peripheral zone, the

hyalomere, and a darker-staining central zone rich in granules, called the

granulomere.

A thin layer of glycocalyx surrounding the platelet plasmalemma is

involved in adhesion and activation during blood coagulation.

Platelets granules

It has Electron-dense delta granules (δG), 250-300 nm in diameter, contain

ADP, ATP, and serotonin (5-hydroxytryptamine).

Alpha granules (αG) are larger (300-500 nm in diameter) and contain platelet

derived growth factor (PDGF), platelet factor 4, and several other platelet-

specific proteins.

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The role of platelets in controlling blood loss (hemorrhage)

1- Primary aggregation: Disruptions in the microvascular endothelium, allow

the platelet glycocalyx to adhere to collagen in the vascular basal wall, forming

a platelets plug

2- Secondary aggregation: Platelets in the plug release a specific adhesive

glycoprotein and ADP, which induce further platelet aggregation and increase

the size of the platelet plug.

3-Blood coagulation: During platelet aggregation, fibrinogen from plasma, von

Willebrand factor and other proteins released from the damaged endothelium,

and platelet factor 4 from platelet granules promote the sequential

interaction(cascade) of plasma proteins, giving rise to network of fibers trapping

red blood cells and more platelets to form a blood clot, or thrombus

4-Clot retraction: The clot that initially bulges into the blood vessel lumen

contracts slightly due to the activity of platelet-derived actin and myosin.

5-Clot removal: Protected by the clot, the endothelium are restored by new

tissue, and the clot is then removed, mainly dissolved by the proteolytic enzyme

plasmin

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Hematopoeisis

Hematopoiesis or hemopoiesis is the production of blood cellular components.

Blood cells are produced in hemopoietic tissues, of which there are two: red

bone marrow, found in flat and irregular bones, and lymphatic tissue, found in

the spleen, lymph nodes, and thymus gland.

Red bone marrow fills the head of the femur, and a spot of yellow bone marrow is visible in

the center. The white reference bar is 1 cm.

In developing embryos, blood formation occurs in aggregates of blood cells in

the yolk sac, called blood islands. As development progresses, blood formation

occurs in the spleen, liver and lymph nodes. When bone marrow develops, it

eventually assumes the task of forming most of the blood cells for the entire

organism. However, maturation, activation, and some proliferation of lymphoid

cells occurs in the spleen, thymus, and lymph nodes.

In children, hematopoiesis occurs in the marrow of the long bones such as the

femur and tibia.

In adults, it occurs mainly in the pelvis, cranium, vertebrae, and sternum (flat

bones).

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In some cases, the liver, thymus, and spleen may resume their hematopoietic

function, if necessary. This is called extramedullary hematopoiesis. It may

cause these organs to increase in size substantially. During fetal development,

since bones and thus the bone marrow develop later, the liver functions as the

main hematopoetic organ. Therefore, the liver is enlarged during development.

Blood-cell development progresses from a hematopoietic stem cell (HSC),

which can undergo either self-renewal or differentiation into a multilineage

committed progenitor cell: a common lymphoid progenitor (CLP) or a common

myeloid progenitor (CMP).

These cells then give rise to more-differentiated progenitors, comprising those

committed to two lineages that include T cells and natural killer cells (TNKs),

granulocytes and macrophages (GMs), and megakaryocytes and erythroid cells

(MEPs).

Ultimately, these cells give rise to unilineage committed progenitors for B cells

(BCPs), NK cells (NKPs), T cells (TCPs), granulocytes (GPs), monocytes

(MPs), erythrocytes (EPs), and megakaryocytes (MkPs).

Growth factors are required for the survival and proliferation of hematopoietic

cells at all stages of development (see figure below). Of the factors that affect

multipotential cells, eryhthropoietin (EPO), thrombopoietin (TPO), steel factor,

Fms-like tyrosine kinase 3 (FLT3) ligand, granulocyte–macrophage colony-

stimulating factor (GM-CSF), interleukin-2 (IL-2), interleukin-3 (IL-3), and

interleukin-7 (IL-7) are the best characterized. Each of these proteins supports

the survival and proliferation of a number of distinct target cells. Collectively,

these growth factors are termed hematopoietic growth factors.

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In the figure below, cytokines and growth factors that support the survival,

proliferation, or differentiation of each type of cell are shown in red. For

simplicity, the three types of granulocyte progenitor cells are not shown; in

reality, distinct progenitors of neutrophils, eosinophils, and basophils or mast

cells exist and are supported by distinct transcription factors and cytokines (e.g.,

interleukin-5 in the case of eosinophils, stem-cell factor (SCF) in the case of

basophils or mast cells, and G-CSF in the case of neutrophils).

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Immunophenotype

The cluster of differentiation (CD) molecules are cell surface markers useful for

the identification and characterization of leukocytes. The CD system is

commonly used as cell markers in immunophenotyping, allowing cells to be

defined based on what molecules are present on their surface. While using one

CD molecule to define populations is uncommon (though a few examples exist),

combining markers has allowed for cell types with very specific definitions

within the immune system.

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Hematopoietic stem cell transplantation

This is a therapeutic procedure in which normal hematopoietic stem cells

(CD34+) from an appropriate donor are transferred to the patient having

defective or diseased marrow to re-constitute normal hematopoiesis.

Types of hematopoietic stem cell transplantation (HSCT)

o Allogenic HSCT

Hematopoietic stem cells are obtained from a donor following HLA (human

leukocyte antigen) typing.

o Autologous HSCT

Stem cells previously collected from the recipient are infused back to the

same patient.

Sources of hematopoietic stem cells

1. Peripheral blood stem cells mobilization and harvesting

Although HSCs are known to circulate in peripheral blood, their very

small number preclude their use for transplantation. Administration of

recombinant hematopoietic growth factor (mainly G-CSF) mobilises

HSCs from bone marrow and enhances their subsequent yield from

peripheral blood. The yield can be evaluated by mononuclear cell count

and CD34+ cells count.

2. Bone marrow harvesting

Up to 100 ml of bone marrow can be aspirated from the donor from

several sites in the iliac crest. Aspirated marrow is harvested in a bag (or

tubes) containing acid citrate dextrose solution.

3. Umbilical cord blood

Umbilical cord blood is an important and readily available source of

hematopoietic stem cells (Stem cell banks). However the number of stem

cells in the cord blood is limited, and therefore, most of the successful

cord blood transplantations have been done in small children.

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