2

Presentation Out line

1. Objectives

2. Introduction

3. Blood volume and constituent

4. Plasma constituents

5. Erythrocytes and blood grouping

6. Leukocytes and immune responses

7. Platelets and Hemostasis

3

1.Objectives

At the end of this session, students will able to:

List the functions of blood

Appreciate blood composition

Understand the structure and functions of RBCs,

WBCsand platelets.

2. Introduction

The Only Fluid tissue in the body.

Specialized type of connective tissue in which formed

elements are suspended in non living fluid matrix called

plasma.

4

5

3. Blood volume and constituent

Physical Characteristics of Blood

viscosity: sticky opaque fluid, due to the presence of

RBCs(sticky and thick), Viscosity (thickness) = 4 – 5.

color :

o Scarlet red-high oxygen

o Dark red-poor oxygen

6

PH: 7.35-7.45

Temperature: Blood temperature is slightly higher than

body temperature.

Blood volume: 5–6 L for males; 4–5 L for

females. Blood accounts for approximately 8% of body

weight.

7

The amount of blood varies with body size, changes in

fluid concentration, changes in electrolyte concentration,

and amount of adipose tissue.

Density ( specific gravity): Refers to the weight of blood

compared to water.

Specific gravity of H20 is taken as 1(i.e.,1 ml of H2O

weighing 1 gm at 4 oc).

8

Male:1.052-1.063

female:1.050-1.058 ( i.e.,1 ml of whole blood

weighing 1.060gm).

Osmolarity = 300 mOsm or 0.3 Osm, reflects the

concentration of solutes in the plasma.

Salinity = 0.85%, Reflects the concentration of NaCl in the

blood.

9

Functions of blood

Blood performs a number of functions dealing with:

1. Substance distribution (Transportation )

2. Regulation of blood levels of particular

substances

3. Body protection

10

Cont’d……

1.Distribution (Transports )

o Oxygen from the lungs and nutrients from the

digestive tract to the tissues .

o Metabolic wastes from cells to the lungs and kidneys

for elimination

o Hormones from endocrine glands to target organs

Cont’d….

2.Regulations

o Appropriate body temperature by absorbing and distributing

heat to other parts of the body

o Maintaining body PH in the body tissues using buffer system.

o Maintaing adequate fluid volume in the circulatory volume.

11

3. protection

o Hemostasis

Activating plasma proteins and platelets.

Initiating clot formation when a vessel is broken.

o infection : Synthesizing and utilizing antibodies.

Activating complement proteins. Activating WBCs to

defend the body against foreign invaders.

Composition of Blood

2 major components

Liquid = plasma (55%)

Formed elements (45%)

Erythrocytes / red blood cells (RBCs)

Leukocytes / white blood cells (WBCs)

Platelets, fragments of megakaryocytes in marrow

( thrombocytes ).

14

Components of Whole Blood

Withdraw blood and place in tube

1 2 Centrifuge

Plasma (55% of whole blood)

Formed elements

Buffy coat:leukocyctes and platelets(<1% of whole blood)

Erythrocytes(45% of whole blood)

15

Cont’d……

Plasma -55%

Buffy coat

RBCs =45%

Plug

Capillary tube

16

4. Plasma constituents

Blood plasma : straw colored sticky fluid, includes:

Water = 90-92%.

Proteins = 6-8%. Albumin,globilin and clotting proteins

Organic nutrients – glucose, carbohydrates, amino acids

Electrolytes – sodium, potassium, calcium, chloride,

bicarbonate.

Non protein nitrogenous substances – lactic acid, urea,

creatinine.

Respiratory gases – oxygen and carbon dioxide.

Plasma proteins

Albumins:

accounts 60% of Wt,Most abundant plasma protein.

Carrier to shuttle molecules through the circulation

17

Important blood buffer.

maintain osmotic pressure of the blood.

Globulins : Accounts 36% of the plasma protein.

α and β globulins have role in transport.

γ globulins are in immuno globulins( IgG, IgA).

Clotting proteins : account for 4%

‾ Fibrinogen and Prothrombin.

Serum

Plasma with clotting factors removed, yellowish color. 18

19

Serum: Plasma with clotting

factors removed, yellowish color.

Determined by means of

electrophoresis

Cont’d……..

Formed elements

Comprise 45% of blood

Erythrocytes, leukocytes, and platelets make up the formed

elements

Only WBCs are complete cells

RBCs have no nuclei or organelles, and platelets are just cell

fragments.

20

Cont’d…….

Most formed elements survive in the bloodstream

for only a few days.

Most blood cells do not divide but are renewed by

cells in bone marrow.

21

Blood

(4.8%) (95.1%) (0.1%)

Plasma

Hormones

MonocytesBasophilsEosinophilsNeutrophils

(54–62%) (1–3%) (<1%) (3–9%) (25–33%)

GlobulinsAlbumins

(92%) (7%)

N2 O2 CO2

Platelets Red blood cells Proteins Nutrients Gases

45% 55%

WastesWaterWhite blood cells Electrolytes

Vitamins

Lymphocytes Fibrinogen

Formed elements

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

24

5. Red Blood Cells ( RBCs)

Shape: flexible biconcave cell that is thinner at the center

and thicker at the edges

Diameter: ~ 7.5 um

- Mature, No nucleus and organelles.

Has greater surface area/volume ratio, Therefore, can bend

and twist to pass through the narrow capillaries very easily.

25

The major forms of cell in the blood.

The average number of RBCs are 5 million/ mm3

blood or 5 x106 / μl of blood.

Lack of mitochondria.

Each RBC contains 280 million haemoglobin.

Half-life app.120 days.

26

27

Hematocrit (Hct) / Packed cell volume (PCV)

Hematocrit (Ht): is the percentage by volume of packed RBC.

Procedure :

1. Centrifuge of un –coagulated blood at a high speed (10-15 min).

2. RBC precipitate down to the bottom.

3. The plasma portion remains floating.

The cells that settle down to the bottom (mainly RBCs) form the

hematocrit or packed cell volume (PCV).

28

Normal value:

Males: 47% ± 5%

Females: 42% ± 5%

HCT increase in

polycythemia and

dehydration states.

Decrease in anemia

Function of RBCs :

1. Carries hemoglobin that in turn transports respiratory

gases (O2 and CO2).

2. Carbonic anhydrase (CA): An enzyme located in RBC

membrane.

CO2 + H2O CA H2CO3 = HCO-3 + H+

CA increases the rate of this reaction 5000 fold.

Good to transport CO2 from the tissues to the lung very fast.

Haemoglobin : Consists of globulin and heme.

Globulin

Two alpha(α) chain polypeptide

Two beta(β) chain polypeptides

Heme: Each heme is present in one peptide chain and

contains an iron {Fe++} that combines reversibly with one

molecule of O2. 30

Each polypeptide has one heme group, each heme with

Fe2+ carries one O2 molecule, total = 4- O2 molecules

Are carried with in Hb molecule.

1g Hb binds with 1.34 ml O2

15g Hb/dl x 1.34 ml O2 = 20.1 ml O2/100 ml blood.

31

32

33

The polypeptide chain (the Globin unit) determines the

physical characteristics of the Hb-molecule. Thus, there exists:

a. Adult Hb (Hb A): 2α + 2β

b. Fetal Hb (Hb-F): 2α + 2γ

c. Sickle cell (Hb-S): glutamic acid is replaced by valine

at Beta- chain so on.

34

Saturation of Hb: refers to the number of O2 molecules

combined with Hb.

The Hb molecule combine maximally with 4 molecules of O2

in a cascade manner(100%saturated),by Oxygenation reaction,

each binding facilitates further binding of O2.

50 % saturation: means that Hb binds with 2 molecules of O2.35

Hematopoiesis/ hemopoieisis

RBC and other blood cells are produced in the Bone

marrow.

All cells emerge from undifferentiated (uncommitted ) stem

cells in the Bone marrow.

Stem cells: All formed elements derived from

single population36

37

Stem cells: All formed elements derived from single population

Proerythroblasts : Develop into red blood cells

Myeloblasts: Develop into basophils, neutrophils,

eosinophils

Lymphoblasts: Develop into lymphocytes

Monoblasts: Develop into monocytes

Megakaryoblasts: Develop into platelets

Hematopoiesis

Erythropoiesis.

A. Embryonic life: RBC are produced in the liver, spleen

and lymph nodes.

B. Infants (5 years old): Red bone marrow of all cells.

C. Adults (after age 20): Membranous bones like

ribs, sternum, vertebrae and pelvic bones.

- But not in long bones like femur or tibia (fat).

39

40

Stem cells differentiate to produce committed stem cells

called hematocytoblasts that in turn produce :

1. Proerythroblast: where Hb synthesis begins, big nucleus.

2. Basophile erythroblast: cell divide, continues

3. Polychromatophil erythroblast: Hb synthesis increases

and fills the cytoplasm, nucleus size decreases.

4. Ortochromatic erythroblast: Nucleus decreases.

5. Reticulocytes: Contains Hb, no nucleus and the cell is

expelled from the bone to circulation.

6. Erythrocytes: Mature form of RBC without nucleus,

filled with Hb.

41

42

Physiological Mechanism

1. Low oxygen(Hypoxia ) that occurs in the kidney cells.

2. Kidney then produce a hormone called erythropoietin.

3. Erythropoietin is transported by the blood to bone marrow.

4. Bone marrow produces and releases a increased RBC .

Increased or adequate O2 then blocks the formation of more RBC.

44

45

Organs involved in erythropoiesis

Kidney : Erythropiotein

Liver : Store protein, vit.B12& folic acid ,Synthesize globin ,

Produce Erythropiotein 10%.

Bone Marrow: Site of RBCs formation

Stomach: intrinsic factor.

Small Intestine: absorption of Iron, vitamins, and amino acids.

46

47

Substances necessary for RBC maturation

A. Vitamin B12: requires intrinsic factor for absorption

- Important for DNA synthesis and thus for cell division.

Deficiency of Vit B12 : Megaloblastic anemia.

Characterized : macrocytic cells (big Hb in

cytoplasm) Because of their big size, the cells rupture

when passing through the capillary wall.

48

Insufficient of intrinsic factor causes , Maturation failure

for Vit. B12 is call pernicious anemia.

B. Folic acid: important in DNA synthesis.

c. Iron: Necessary for RBC formation.

D.Trace elements : (Co, Vit. Copper etc).

49

Destruction of RBCs The absence of nucleus in erythrocytes prevents them

from synthesizing proteins and other important

substances necessary for survival.

The cells become weak and fragile and die after about

120 days.

50

The older red cells are phagotizised by macrophage cells of

the reticuloendothelial system that are located in the liver,

spleen, and bone marrow cells.

The macrophages release the Hb-molecule that is broken down

into:

a. its protein part (Globin) and

b. Heme part

51

Steps in the destruction of RBC:

1. RBC = Globin + Heme

2. Globin = Broken to AA’s > used for protein synthesis

3. Heme = Fe2+ + poryphrine rings.

4. Fe 2+ = stored in the liver > used for new Hb synthesis.

5. Pyrol rings > oxidation to green pigment called Biliverdin and

later reduced to bilirubin.

52

a. Bilirubin + serum albumin > reach liver.

b. Bilirubin conjugates with glucuronic acid in liver.

C. Liver releases bilirubin as bile to Small intestine.

D. Bacteria's change bilirubin into:

Strrcobilinogen > stercobilin , feces (brown color).

Uribilinogen > Urobilin > Urine (yellow).

Life Cycle of a Red Blood Cell

54

55

Clinical correlations: Anemia

A decrease in;

1- RBC number or/and

2- Hb content Below the normal for that sex and age.

1.Decreased RBC Number.

- Blood loss : e.g, hemorrhage

- A plastic anemia: bone marrow destruction (X-ray)

- Maturation failure anemia: pernicious anemia.

56

Microcytic Hypochromic Anemia: Low levels of

hemoglobin in RBCs due to chronic blood loss

resulting in low Fe2+ levels in newly produced

RBCs.

2. Hemolytic Anemia: Different abnormalities of

RBCs that make RBCs fragile and rupture easily.

57

Hereditary Spherocytosis: RBC develop as small

spherical cells . These spherical cells easily rupture by

slight compression.

Sickle-cell Anemia:

Genetic mutation causing abnormal beta chains.

HgS exposed to low O2 concentrations, it precipitates into

long crystals that cause the cells to become sickle-shaped.

Effects of Anemia

• Due to decrease O2 supply to tissues.

1- Fatigue, muscle weakness

2- Mental effects: lack of concentration and dizziness , even Faintining

3- CVS effects: tachycardia, palpitation, heart failure if not treated

4- nausea & anorexia

5- Retarded growth in children

58

Polycethemia

is abnormal increase of RBC in the circulation.

Two types

1. Polycethemia Vera (8-9 million)

Tumerous or cancerous production causes highly engorged blood.

genetic mutation in the hemocytoblastic cell line that increases

RBC production. Hematocrit values can reach 70%

59

60

2. Secondary Polycethemia;

Mostly Physiologic

Increase in RBC production due to hypoxic tissues.

e.g. high altitudes.

Effect of polycethemia on the circulatory system

1. Increased viscosity causes sluggish blood movement.

2.thrombosis and obstruction of different blood vessels.

3.decreased blood flow to tissues and Decreased delivery of O2 to

tissues.

4.Hct increases and so blood volume, blood pressure and work of

the heart increases.

RX: aim to remove RBC by; phlebotomy, blood donation61

62

6.Blood Groups

Erythrocytes contain genetically determined surface

antigens( agglutinogens).

Blood plasma contains antibodies(agglutinins )that react

with specific antigens.

Blood is named according to surface antigens that are

present.

63

Types of blood group

In humans, there are two known blood groups that

are clinically important:

a. The ABO-Blood groups

b. The Rh- Blood group factors

64

ABO Blood Group In the ABO system, blood is classified primarily on the

Basis of the A and B antigens present on the surface of

red blood cell membranes (erythrocytes).

Secondly, blood is classified on the basis of the naturally

occurring antibodies (agglutinins) in the serum .

A person whose red cells possess the A -antigen has anti-B

antibody in his serum and is classified as Blood group A.

If B antigen is present in the Red cell membranes, Anti-A

antibody is present in his serum and the person is designated

as Blood group B.

If Both AB antigens are present on Red cells, then he has

no antibody, so is AB blood group.

If No antigens are present on red cells, he is O Type and

has both anti A and B antibody in his serum. 65

ABO Blood Group

4 blood types – A, B, AB, O

Types are identified by antigens located on the RBC surface. 66

67

68

ABO-blood group

Typing and cross-matching – process by which blood

type is identified and donor blood is tested for possible

transfusion.

Transfusion:

Type O is a Universal Donor.

Type AB is the Universal Recipient.

69

70

Blood group O

Universal Donor.

No antigens on their cell-membrane surfaces and therefore can

not agglutinate if transfused to any blood types.

Receive only from persons with blood group “O” only

because , they have anti- A & anti-B antibodies in the plasma.

71

Blood group AB

Universal recipient

they have no antibodies in their blood to cause

agglutination reactions.

Have antigen A and B, AB can donate blood only to a

person with blood AB, not to other.

72

Method of blood typing

Procedures:

1. On a slide at opposite ends , drops of anti- A antibody and

Anti-B antibody are added at opposite sides.

2. 2-3 drops of Blood (RBC’s) are added on the prepared

antibodies and changes for agglutination are observed after

a few minutes.

A. If agglutination occurs on anti-A antibody (sera), then the

blood is Blood Group A.

B. If agglutination occurs on anti-B antibody , the blood is

Blood Group B.

C. If there is agglutination in both A and B-antibodies, then it

is Blood Group AB.

D. If no agglutination occurs, then it is Blood Group O.73

74

75

Agglutination Reaction

Donators and Recipients

Donators 1. O can donate blood to group A, B, AB, and O2. A “ A & AB only3. B “ B & AB only 4. AB “ AB only

Recipients 1. O can receive blood from group O only2. A “ A & O only3. B “ B & O only4. AB “ A, B, O, & AB

76

RH Blood Group

Named after Rhesus monkey.

Consists of over 50 related antigens, the most clinically

significant is D,C,E,c ,d and e.

The type D antigen is more antigenic and widely prevalent in

the population.

Rh+, having type D antigen, 85% of the population

Rh-,lack of type D antigen , 15% of the population 77

78

• A person with Rh negative(-)blood does not have Rh

antibodies naturally in the blood plasma.

• If they receive blood that is Rh positive (+) , antibodies

form but not a problem.

• The second exposure can produce a transfusion reaction

(Hemolysis and possible kidney damage).

Rh incompatibility

Father Rh + = Rh + means he has D antigen on his RBC

membrane.

Mother Rh- = No Rh factor(no D antigen).

Marriage:

1. Rh+ father X Rh- mother = Rh + fetus.

2. During birth through placenta , Rh+ blood (antigens) of the

fetus leak (enter) to mothers blood and sensitizes her.79

80

3. Mother ‘s blood produces anti-Rh antibodies (anti-D

antibodies ) against the Rh+ blood.

4. During the 2nd pregnancy and there after, the Anti-Rh+

antibodies (agglutinins) enter into the fetus and agglutinate or

hemolyze the RBC’s the fetus.

This type of hemolytic disease is called

Erythroblastosis fetalis.

If the baby is born alive from the incidence, then there is a

higher risk of being Anemic and jaundiced.

81

82

Prevention

• Shortly after each birth of an Rh+ baby, the mother is given

an injection of anti-Rh antibodies (or Rhogam).

These passively acquired antibodies destroy any foetal

cells that got into her circulation before they can elicit an

active immune response in her.

Bio 130 Human Biology

19-84

Erythroblastosis Fetalis

85

7. White Blood Cells( Leukocytes ) and immune response

86

Properties The only formed element that are complete cells.

Normal number: 4000-10,000 / mm3 of blood

Mobility: Are highly mobile and reach tissue fluids.

When infection occurs, WBC increase in number e.g.,

Neutrophils.

Life span : Many (not all) live only a few days, may be b/s of

their engagement with pathogens.

Grouped into two main categories.

Granulocytes : contain specialized membrane-bound

cytoplasmic granules.

Contain cytoplasmic granules that stain specifically

(acidic, basic, or both) with Wright’s stain

Are larger and usually shorter-lived than RBCs.

Have lobed nuclei, Are all phagocytic cells.

include neutrophils, eosinophils and basophiles. 87

88

Agranulocyte : Lack obvious granules; Lymphocytes and Monocytes.

Granulocytes

1.Neutrophils: Make up 60 to 70% of WBC’s.

Diameter of 10-15 μm, Phagocytic

First to arrive at infections.

Nucleus 2-5 lobes (increase with cell age)

Increase: stress, burns and bacterial infections.

Decrease: Radiation exposure, B12 deficiency.89

2.Eosinophils : Account for 1-4% of WBCs

10 –12 um in diameter, Nucleus 2–3 lobed

Increase: allergic reactions, parasitic infections and

autoimmune disease.

1. Kill parasitic worms

2. Destroy antigen-antibody complexes.

3. Inactivate some inflammatory chemical of allergy (histamine). 90

3. Basophils: Account for 0.5% -1% of WBCs.

Liberate heparin and histamines during allergic reactions.

Intensify inflammatory response

Increase: Allergic reactions, leukemia, cancers,

hypothyroidism.

Decrease: Pregnancy, ovulation, stress, hyperthyroidism

91

Agranulocyte

1. Lymphocytes: Make up 20 to 25% WBC’s

Small: 6-9 um in diameter, Large: 10-14 um, Nucleus is round

or slightly indented.

B cells produce antibodies.

T cells attack viruses, cancer cells, and transplanted tissues

Natural killer cells attack infectious microbes and tumor cells.92

2. Monocytes

Account for 4-8% of WBCs and the largest WBCs.

They leave the circulation enter tissue and differentiate

into macrophages.

Activate lymphocytes to mount an immune response.

Phagocytize bacteria, dead cells, and other debris.

93

94

95

mobility through the tissues

1. Diapedesis: WBC Squeeze out through the capillary pore (e.g.

Neutrophils, Monocytes ).

2. Amoeboid motion: Produce pseudopodia and reach the microbes in

the tissues

3. Chemotaxis: WBC are attracted by chemicals or toxins produced by

the microbe or inflamed tissues

4.Phagocytosis: engulfing and destroying e.g., Neutrophils,

macrophages .

96

Clinical correlation

Leukemia (increased WBC No):

cancerous production of WBC.

These occurs:

a. in the bone marrow

b. in the lymph.

Their increased production takes the space of platelets &

RBC causing anemia + impaired blood clotting 97

Leucopenia: Decreased production of WBC

- Bone marrow stops producing them

- Drug poison, X-rays

98

99

Practical hematology/leukocyte differential

Lists the different percentages of leukocytes

↑Neutrophils : Bacterial Infection

↑ Lymphocytes: Viral Infection

↑ Monocytes: Chronic Infection

↑ Basophils: Allergic Rxns

↑Eosinophils: parasitic infections

8. Platelets and Hemostasis

100

Platelets/Thrombocytes

Small, non nucleated (anucleated), round/oval cells/cell

fragments.

Their size ranges 1-4-m in diameter.

The cytoplasm stain pale blue and contain many pink

granules.

They are produced in the bone marrow by fragmentation of

megakaryocytes, which are large and multinucleated cells.101

• Their primary function is preventing blood loss from

hemorrhage by forming a platelet plug

• Normal value – 150,000 to 300,000/mm3.

Platelets have a life span of approximately 10 days.

Senescent platelets are removed by the spleen.

102

Attracted to hemorrhage.

Plugs leaks.

Promotes constriction of blood vessel.

Triggers inflammation.

Initiates clotting.

103

104

• Hemostasis refers to the stoppage/arresting of bleeding.

• Actions that limit or prevent blood loss include:

• Blood vessel spasm

• Platelet plug formation

• Blood coagulation

Hemostasis

105

Blood coagulation

Triggered by cellular damage and blood contact with

foreign surfaces.

• A blood clot forms

• Causes the formation of clot via a series of reactions

which activates the next in a cascade

• Occurs extrinsically or intrinsically. 107

Extrinsic clotting mechanism

• Triggered when blood contacts damaged blood vessel

walls or tissues.

• Chemical outside of blood vessel triggers blood

coagulation.

• Triggered by tissue thromboplastin (factor III) (not

found in blood).

•

•

• .

• A number of events occur that includes factor VII, factor X, factor V,

factor IV, and factor II (prothrombin).

Intrinsic clotting mechanism

Triggered when blood contacts a foreign surface

• Chemical inside blood triggers blood coagulation

• Triggered by Hageman factor XII (found inside blood)

• Factor XII activates factor XI which activates IX which joins with

factor VIII to activate factor X.

110

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Tissue damage

Releases

Factor Vll

Factor X

(Ca+2)

(Ca+2)

Activates (Ca+2)

Converts

Converts

(Ca+2)

(Ca+2)

Activates

Activates

Factor V

Fibrin

Factor lX

Factor Xl

Factor X

Activates

Activates

Activates

Activates

Activates

Hageman Factor Xll

Factor V

StabilizesFactor Xlll

Extrinsic ClottingMechanism

Tissue thromboplastin(Factor lll)

Blood contactsforeign surface

Intrinsic ClottingMechanism

Prothrombinactivator

Prothrombin(Factor ll)

Thrombin(Factor lla)

Fibrinogen(Factor l)

Fibrinclot

Factor Vlllplatelet phospholipids

111

Fate of Blood Clots

• After a blood clot forms it retracts and pulls the edges of a

broken blood vessel together while squeezing the fluid

serum from the clot.

Platelet-derived growth factor stimulates smooth muscle

cells and fibroblasts to repair damaged blood vessel walls.

Plasmin digests the blood clots.

112

113

Clinical correlations

1. Hemophilia A: Deficiency of Factor VIII accounts for

85% cases. Almost exclusively in males.

Females are usually carriers, caused by a gene mutation

on the “X” chromosome. Occurs in about 1/10,000 male

births

Other Hemophilias account for another 15% ,Hemophilia B (Factor

IX),Hemophilia C (Factor XI) and Hemophilia D (Factor XII)

114

2.Thrombocytopenia: Abnormally low levels of platelets.

Usually below 50,000/ μl of blood.

3.Thrombus: Abnormal clot that develops in a blood

vessel.

4.Embolus: Free thrombic clots carried in the blood that

usually get caught in arterioles in the brain, kidney, and

lungs.

Practical hematology/Tests for Bleeding

1. Bleeding time : The time interval that takes between start of

bleeding (oozing) until arrest of blood. Normal duration : 3-6 min

2. Clotting time: The duration of time it takes for the blood to clot

(normal duration is 3-8 min).

3.Prothrombin time: Deals with the duration of formation of

Prothrombin after addition of oxalate and Ca2+ ions to the blood.

116

Reading assignment on immunity