CHAPTER 11: IMMUNITY · 2021. 1. 17. · Humoral immune response The branch of adaptive immunity that involves the activation of B cells and that leads to the production of antibodies,

Biology Student’s Companion Resources SB025

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CHAPTER 11: IMMUNITY

SUBTOPIC :11.1 IMMUNE RESPONSE

LEARNING OUTCOMES: (a) Define immunity and state the types of immunity

(b) Describe the general structure of antibodies and state the classes based on its

structure.

(c) State the roles of lymphoid organs in immunity such as:

i. Thymus

ii. Spleen

iii. Tonsil

iv. Lymph nodes

v. Bone marrow

(d) Explain the various types of antigen and antibody interactions:

i. neutralization

ii. opsonization

iii. activation of complement system and pore formation.

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/KEY POINT EXPLANATION NOTES

a. Immunity and

types of

immunity

Immunity:

The ability to recognize and destroy foreign or dangerous macromolecules

(Solomon 10th Ed, 2015).

The state of having sufficient biological defenses to avoid infection, disease or

other unwanted biological invasion by foreign organism or other foreign

substance

Types of immunity:

a) Innate immunity

b) Adaptive immunity

INNATE IMMUNITY (all animals)

Recognition of traits shared by

broad ranges of pathogens, using a

small set of receptors

Rapid response

ADAPTIVE IMMUNITY (vertebrates only)

Recognition of traits specific to

particular pathogens, using a vast

array of receptors

Slower response

Barrier defenses: Skin Mucous membranes Secretions

Internal defenses: Phagocytic cells Natural killer cells Antimicrobial proteins Inflammatory response

Humoral response: Antibodies defend against infection in body

fluids.

Cell-mediated response: Cytotoxic cells defend against infection in body

cells.

Pathogens (example: fungi,

bacteria and viruses)


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b. General

structure of

antibodies and

classes based on

its structure.

Antibody:

A specific protein (immunoglobulin) that recognizes and binds to specific

antigen which produced by plasma cells.

Structure of antibodies:

Basic shape of antibody

• Y shaped • 4 polypeptide chains:

✓ 2 identical light chains ✓ 2 identical heavy chains

• Linked by disulphide bridge • Each chain contains:

✓ variable region (V) ✓ constant region (C)

• The constant region of heavy chains: ✓ form the base of antibody molecules ✓ determines the way antibody is secreted & how it is distributed in

body fluid

✓ Eg: circulate in body fluid or bind to membranes of mast cells (mast

cells are cells found in connective tissue that store histamine,

inflammatory signaling molecule)

• The variable region of the light and heavy chain: ✓ determine the specificity of an antibody molecules ✓ amino acid sequence varies extensively from one B cell to another. ✓ part of heavy-chain V region and a light-chain V region form as

asymmetric binding site for an antigen.

• Each antibody has 2 antigen-binding sites ✓ Form by the free tips end of both variable region


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Classes of antibody:

i. IgD

• Found in blood and lymph and on surfaces of B cells • Single monomer • Function: act as antigen receptors on the surfaces of

B cells ✓ Stimulating the differentiation of B cells into

plasma cells and memory cells ii. IgE

• Bind to mast cells and basophil and stimulate them

to produce histamines • Single monomer • Function: responsible for allergic reaction but useful

against parasitic worms

iii. IgG

• Temporary protection to newborn. • single monomer • Easily cross the walls of blood vessel and enter tissue

fluid • e.g.: maternal IgG antibodies can cross the placenta

and confer passive immunity to fetus

iv. IgM

• Primary antibody response. • Pentamer: ✓ Large size ✓ 5 monomers held together by

polypeptide called J (joining) chain • First antibodies to appear in response to initial

exposure to an antigen

v. IgA

• Common in mucous membranes and in

body secretions: mucus, saliva, tears, breast

milk • Most abundant in body (IgG most abundant

in serum) • Dimer: 2 monomers held by J chain • Functions: prevent the attachment of

pathogens to mucosal surfaces


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c. Roles of

lymphoid

organs in

immunity:

i. Thymus

ii. Spleen

iii. Tonsil

iv. Lymph

nodes

v. Bone

marrow

i. Thymus

• Gland that produces several hormones that are important in developing

& maintaining immune defenses ✓ e.g.: thymosin - promotes the development and maturation of

lymphocyte • Function: Site where the lymphocyte cells mature ✓ Lymphoid stem cells migrate to the thymus ✓ Subsequent divisions produce daughter cells that mature into T

cells

ii. Spleen

• Contains lymphocytes and macrophages • Functions: ✓ Remove abnormal blood cells & other components by phagocytosis ✓ Store iron recycled from erythrocytes ✓ Initiate immune responses by B cells and T cells in response

to antigens in circulating blood

iii. Tonsil

• Large lymphoid nodules in the walls of pharynx • Functions: ✓ Trap bacteria and viruses which were breathe in ✓ Antibodies and immune cells kill the pathogen ✓ Prevents infections in the lung and throat

iv. Lymph nodes

• Small lymphoid organs • Greatest number: neck, armpit, groin • Function: filter and purifies lymph before it reaches the veins


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v. Bone marrow

• Site of origin of all types of blood cells • Hemocytoblasts in bone marrow divide to form ✓ myeloid stem cells and ✓ lymphoid stem cells

• Myeloid stem cells divide to form: erythrocyte, platelets, granulocytes,

monocytes

• Lymphoid stem cells divide to form lymphocytes

d. Various types

of antigen and

antibody

interactions:

i. neutralization

ii. opsonization

iii. activation of

complement

system and

pore

formation

Antigen:

• Any foreign molecule that elicits an immune response by binding to

receptors of B cells or T cells. • Usually protein, glycoprotein or polysaccharide • Epitope/antigenic determinant sites: a small, accessible region of an

antigen to which an antigen receptor or antibody binds • An antigen may have several different epitopes • Each epitope is recognized by a different antibody • Different antibodies can recognize distinct epitopes on the same antigen

Antigen and antibody interactions:

• An antigen-antibody complex forms when an antibody molecule binds to

its corresponding antigen molecule • Once the two molecules are in position, hydrogen bonding and other weak

chemical forces lock them together • Antigen covered with antibodies attract eosinophils, neutrophils and

macrophages • These cells then phagocytize the pathogens


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i. Neutralization

• A process in which antibodies bind to

proteins on the surface of a virus. • Viruses and bacterial toxins have

specific sites that must bind to target

regions on body cells before they can

enter or injure those cells. • Antibodies bind to those sites, making

the virus or toxin incapable of attaching

itself to cell. • Thus, neutralizing the virus/toxins.

ii. Opsonization

• A process by which a particulate antigen becomes more susceptible to

phagocytosis by macrophages and

neutrophils. • A coating of antibodies and

complement proteins increase the

effectiveness of phagocytosis. • Some bacteria have slick plasma

membranes or capsules, but

opsonization makes it easier for

phagocytes to hang on onto their prey

before they engulf it. • Phagocytes can bind more easily to antibodies and complement

proteins than they can to the bare surface of a pathogen.

iii. Activation of complement system and pore formation

• When an antibody molecule binds to an antigen, it forms antigen-

antibody complex. • Portion of the antibody change shape exposing areas that bind

complement protein. • This binding activates the complement system. • The activated bound complement molecule then forms membrane

attack complex. • Pores formed in plasma membrane, allowing water and ions to rush in. • The cell swells and lyses, destroy the pathogen.


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SUBTOPIC: 11.2 DEVELOPMENT OF IMMUNITY

LEARNING OUTCOMES: (a) State the two types of immune response.

(b) Explain humoral and cell mediated immune response against infection.

(c) Explain the primary and secondary immune responses.

MAIN IDEAS


a. Two types

of immune

response.

Types of immune response:

i. Humoral immune response The branch of adaptive immunity that involves the activation of B cells and

that leads to the production of antibodies, which defend against bacteria and

viruses in body fluids.

ii. Cell mediated immune response The branch of adaptive immunity that involves the activation of cytotoxic

T cells, which defend against infected cells.

b. Humoral

and cell

mediated

immune

response

against

infection.

Components of humoral and cell mediated immune response:

i. Lymphocytes

Account for 20 – 30 % of circulating leukocytes

(a) B cells

Make up 10-15% of circulating lymphocytes

• Plasma cells ✓ when stimulated, B cells can differentiate into plasma cells

which produce and secrete antibodies

• Memory B cells ✓ Subset of B cells that respond to a previously encountered antigen


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(b) T cells

Approximately 80% of circulating lymphocytes are classified as T

cells

• Cytotoxic T cells ✓ Attack foreign cells or body cells infected by viruses.

• Helper T cells ✓ Stimulate the activation and function of both T cells and B

cells. • Suppressor T cells ✓ Inhibit the activation and function of both T and B cells.

• Memory T cells ✓ Subset of T cells that respond to a previously encountered

antigen

ii. Major histocompatibility complex (MHC) molecule

A host protein that functions in antigen presentation.

• Glycoprotein and antigen that appears in plasma membrane is capable

to activate T cells. • Membrane glycoproteins are called MHC proteins/human leukocyte

antigens (HLAs). • Amino acid sequences and shapes of MHC proteins differs among

individuals. • For T cell to recognize an antigen, the antigen must be bound to

glycoproteins in the plasma membranes of another cell.


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(a) MHC class I molecule

• Present in plasma membrane of all nucleated cell • Pick up small peptides from the surrounding cytoplasm and carry

them to the cell surface • If the cytoplasm contains abnormal peptides or viral proteins, they

soon appear in the plasma membrane • T cells will recognize them as foreign and be activated

(b) MHC class II molecule

• Present only in the plasma membranes of antigen presenting cell

(APC) and lymphocytes • APCs are specialized cells responsible for activating T cell defenses

against foreign cells (including bacteria) and foreign proteins

iii. Cytokines

Signaling proteins that regulate interaction between cells in the immune

system

(a) Interleukin I (IL-1)

• Primary source: macrophages • Activates helper T cells

(b) Interleukin 2 (IL-2)

• Source: Helper T cells. • Activates B cells and cytotoxic T cells

Helper T (TH) cells stimulate the humoral and cell-mediated immune

response

1. The macrophage ingest a microbe or other foreign particle and breaks it

into fragments (foreign antigens)

2. MHC class II (self protein) bind to foreign antigens

3. Display them on the cell surface

4. Helper T cell recognize and bind to the MHC-antigen complex caused

APC release IL-1; and diffuse to the TH cell and stimulate it.

5. Activated TH cell release IL-2. IL-2 makes TH cell itself grow and divide

producing memory cells and additional active TH cells.

6. IL-2 activate B cells and stimulating the humoral immune response

7. IL-2 activate TC cells stimulating the cell mediated immune response


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Humoral immune response

i. Antigen presenting cell @ APC (dendritic cell / macrophage / B cell)

phagocytizes pathogen and degrades it.

ii. Fragment of foreign antigen binds to the class II MHC protein forming

MHC-antigen complex and displayed on the surface of APC

iii. TH cell with its antigen receptor and accessory protein, CD4 binds to the

complex of APC


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iv. APC releases interleukin 1 (IL-

1) to activate TH cell

v. Activated TH cell releases IL-2

vi. TH cell proliferates producing

activated TH cells and memory

TH cells

vii. Once antigens are bound to B

cell receptor (antibody) in the B

cell membrane, the B cell

displays those antigens that

bound to class II MHC

molecule on the surface of its plasma membrane

viii. Activated TH bind to the sensitized B cell.

ix. Activated TH cell will release IL-2 that co-stimulate the sensitized B cell

and trigger its activation.

x. The activated B cell then proliferates or divides mitotically, producing

memory B cells and plasma cells (effector cells).

xi. Plasma cells secretes antibodies.

xii. Antibody-antigen interaction occurs.

xiii. Triggers processes leading to pathogen destruction

xiv. Memory B cells are long-lived cells that can give rise to effector cells if

the same antigen is encountered later in life

Cell mediated immune response

i. Pathogens bearing foreign antigens invade body

ii. Antigen presenting cell @ APC (dendritic cell / macrophage)

phagocytizes pathogen


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iii. Foreign antigen binds to the

class II MHC protein form

MHC- antigen complex and

displayed on surface of APC

iv. TH cell with its antigen

receptor and accessory protein,

CD4 binds with APC

v. APC releases interleukin 1

(IL-1) to activate TH cell

vi. Activated TH cell release IL-2

vii. TH cell proliferate producing activated TH cells and memory TH cells

viii. Activated helper T cell will release IL-2 and the cytotoxic T (TC) cell is

activated.

ix. TC cell also becomes active when encounters an appropriate antigens

bound to class I MHC molecule of infected cell.

x. Activated TC undergoes mitotic division producing active TC cells

(effector cells) and memory TC cells.

xi. Activated TC cell migrates to the area of infection and eliminate cells that

are infected by viruses or other intracellular pathogen.

xii. Fragments of foreign proteins inside of the cell bind with class I MHC

molecules and then displayed on the cell surface. The MHC-antigen

complex is recognized by TC cells.

xiii. TC cells with accessory protein CD8 binds to the MHC molecule, helping

keep the two cells in contact.

xiv. TC cell may:

- release perforin to destroy the target cell’s plasma membrane

and form pores. - Secrete a poisonous lymphotoxin, granzymes to kill the target

cell. - It activates genes in the target cell’s nucleus that program the

cell to die (apoptosis).


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c. Primary and

secondary

immune

responses.

• Results from exposure of B cell to an antigen

• Includes series of cell division, differentiation and antibody production.

• Before stimulation by an antigen, B cells are small lymphocyte.

• After activation, B cell undergoes a series of divisions to produce large

lymphocyte.

• Some enlarge cells become plasma cells to produce antibody.

• Others revert back to small lymphocytes become memory B cells.


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Primary immune response

• Low concentration of antibodies produced at early stage and peak up one to

two weeks after exposure (test on antibody titer).

• Has lag time: slow reaction during 3- 6 days after the exposure.

• B cells specific for that antigen multiply and develop into plasma cells

• Takes 3-14 days to produce enough antibodies to be effective against

antigen.

• Meantime, individual usually develops disease symptoms because the

antigen has had time to cause tissue damage.

• Plasma cells secrete antibody.

• Antibody concentration rise and reach the peak in 10 -12 days.

• IgM is the first antibody produced and later other classes of antibodies are

produced as well.

• Primary response lasts several days or weeks.

• Concentration of antibodies decrease because plasma cell dies.

• Memory B cell left in the body.

Secondary immune response

• The response of immune system to the second infection for same antigen

• Memory cell recognize the same antigen faster.

• Within hours after second exposure: memory B cells proliferate &

differentiate rapidly into plasma cells to produce antibody.

• IgG mainly antibody produced.

• Within 2-3 days, antibody rises steeply, higher than in primary response and

remain high for weeks to months.

• Plasma cells functioning for much longer than in primary response.

• Memory B cell able to recognize antigen for longer period; may persist for

many years and probably for life or the immunity is long lasting.


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Documents

CHAPTER 11: IMMUNITY · 2021. 1. 17. · Humoral immune response The branch of adaptive immunity that involves the activation of B cells and that leads to the production of antibodies,