1 Introduction to Immune System

Prof Ammu K. Radhakrishnan FOUNDATION (Medical)

Learning Outcomes:

You will be able to:

1. describe the functions of the immune

system

2. name the cells of the immune system

3. describe the types of immune

responses that protect host from

infections

4. discuss the different types of

immunity with examples

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Immune System

A body system that can

recognise and dispose

foreign/non-self

Microorganisms

Proteins

Organs/tissues

Other material

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Immune System

The body system that

1. protects host from infections

2. helps host to fight/recover from

infections

3. prevent recurrent infections

4. helps host in healing

5. protects host from cancers

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Immune System: Functions

1. Defence against infections

recognise & destroy harmful

micro-organism

provide long-term protection

against recurrent infections

2. Defence against tumours/

cancers

3. Key role in inflammation,

recovery & healing 24 October 2013

Immune System

A complex body system

Involve many types of:

Cells Molecules

Soluble proteins

Organs Lymphoid organs and tissues

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Immune System: Main

Components

Cells

White blood cells

Leucocytes

Soluble defense proteins

Antibodies

Cytokines

Complement proteins

Others

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Immune System

Deals with microorganisms and

substances that are VERY

SMALL

How “Small is Small”?

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How Small is Small? Germs and the

Immune System: Getting a Sense of

Scale…

Picture the flat end of a pin, that

is about 1 mm across.

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1 mm


Immune System: Getting a Sense of

Scale…

If we placed some bacteria-

eating leucocytes end-to-end

across the flat end of a pin

would have room for 50

leucocytes in a line

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Immune System: Getting a sense of

Scale…

If we did the same for a typical

bacterium

we would have room for 500

bacteria in a line!

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Scale…

If we did the same again for a

typical virus

we would have room for 10,000

viruses in a line!

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Scale…

If we did the same yet again for

an antibody molecule

we would find room for perhaps

100,000 antibody molecules in a

line

that stretched for just 1 mm!

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Phagocyte (WBC) > Bacteria >

Virus > Antibody

All very small

Invisible to the human eye!



Scale…

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Edward Jenner (1749-1823)

Noticed that:

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1

• An attack of smallpox protected against further smallpox disease

2

• Milkmaids who recovered from cowpox were protected from smallpox

Immunisation: Live Microbes

Daring experiment!

Idea of using cowpox

virus to induce

“protection”

(immunity) to

smallpox

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May 1796

inoculated an

8-year old

boy with

cowpox virus

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The boy went

through the

normal

course of

cowpox

disease

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July 1796

Inoculated the

same boy with

small pox

Child did not get

Smallpox! 24 October 2013


First demonstration of using a live not-so virulent form of micro-organisms

To protect against live, virulent form of the same/similar micro-organism

Pathogens shared some structural similarities

“cross-reactivity”

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Louis Pasteur (1888)

Studied the

pathogenesis of

cholera infection

in chicken

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Immunisation: Attenuated

Microbes

Live,

fresh

cholera

culture

DEAD 24 October 2013


Microbes

Dead/weak

“old” cholera

culture

ALIVE ! 24 October 2013


Microbes

ALIVE ALIVE

1. Inject dead, old

cholera

2. Inject live,

fresh cholera

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Microbes

First demonstration of using a weakened micro-organisms

to protect against live, virulent form of the same micro-organism

Attenuated micro-organism

usually alive but greatly weakened

virulence & pathogenicity

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Vaccinations

Immunisation

Many successful vaccinations

were demonstrated

In 18-19th century

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Defense Mechanisms

•Non-specific

•Natural/Native

•Specific

•Acquired

Innate Immune

Responses

Adaptive Immune

Responses

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Innate Immune Response:

General Characteristics

A type of host defense that is always present in healthy individual

Provides the initial protection against infections

1. Block entry of microbes

2. Rapidly eliminate microbe that enter host

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Innate Immune Response:

General Characteristics

Can effectively combat many

infections

Some pathogens have evolved

to resist innate immunity

Need to activate adaptive

immune response

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Adaptive Immune Response: Key

Characteristics

Host defense that is stimulated

by microbes that has invaded

the tissues

Develops more slowly and later

Mediates a more effective

defense against infections

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Invasion by Pathogens

Innate

defense mechanism

Adaptive

defense mechanism

Barriers

Skin

mucous lining

Acid & Enzymes

Inflammation

Phagocytes

Interferon

Complement

Antibody-

mediated

Cell-

mediated

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Immune System

Useful Terminology

Self and non-self

Antigen

Epitope

Antibody/immunoglobulin

Immunity

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What is “SELF”?

NORMAL

CONDITIONS

everything that

is coded within the

genome of an organism

IT IS IN THE GENES!

Rule does

not apply to

autoimmune

conditions

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What is “NON-SELF”?

Not coded for by the self

genome

Examples:

1. harmful micro-organisms

2. foreign proteins

3. transplant organs/tissues

4. incompatible blood transfusions

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Antigens

Substances that can trigger the adaptive arm of the immune system

E.g.

1. Foreign proteins, cells

2. Pathogens

E.g. Bacteria, Virus, Fungus, parasitic worms

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Antigen (Ag)

Substances recognised by:

1. Specific receptors on surface of B- or T- lymphocytes

T-cell receptors (TCR)

B-cell receptors (BCR)

2. Antibody/Immunoglobulin (Ig)

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Epitopes

Antigenic determinants

Areas exposed on antigens

that are recognised by:

1. Antibody (Ig)

2. Lymphocyte receptors (TCR,

BCR)

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Epitopes

Antigen

4 possible

Epitopes 1

2 4

3

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Immunodominant Epitope

The epitope on an antigen to

which there is the highest

immune response to

Useful in the design and

development of recombinant

vaccines

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Immunoglobulin (Ig)

Also known as antibody (Ab)

Defense glycopproteins in

plasma

Produced by Plasma cells

B-lymphocytes will differentiate

into plasma cells when activated

by specific antigens

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Immunoglobulin (Ig): Basic

Structure

Fc Region

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Immunoglobulin (Ig): Classes

5 classes of antibodies

1. IgG

2. IgA

3. IgM

4. IgE

5. IgD

Based on genes utilised

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IMMUNITY

Latin word “immunitas” which

means “Protection”

Does not refer to type of immune

response

There are two types of

protections

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IMMUNITY

PASSIVE ACTIVE

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Passive Immunity

A type of protection that does

not require activation of host’s

immune system

Host immune system is not

activated

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Passive Immunity

How is protection conferred to

individuals?

Transfer of lymphocytes or

antibodies from another

individual who is immune to a

naïve individual

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Passive Immunity

Limited period of protection

Depends on lifetime of transferred lymphocytes or antibodies

Short-lived

Used for rapidly conferring immunity even before individual is able to mount an active immune response

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Passive Immunity: Examples

1. Anti-Rhesus; anti-toxin, anti-

venom,

2. Maternal IgG that crosses the

placenta to the foetus

3. IgA from mother’s milk

Colostrum

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Active Immunity

Requires activation of host’s

immune system

Produce long-lived memory

cells

Those who recover from some

epidemic or infectious diseases

were exempt from further attacks

Long-lived immunity/protection

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Active Immunity: Examples

1. Recovery from infections

2. Vaccinations

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Types of Immune Responses

Blood consist of:

Fluid part (plasma)

Living cells (cells)

Early 18-19th

debates about which component of blood is responsible for the immune response

Cells or fluid?

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Adaptive Immune Response

Antibody (Humoral)

Extracellular microbes/antigens

Cell-mediated

Intracellular microbes/antigens

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Immune Response

Cell-mediated Humoral

Involve

Leucocytes Antibodies

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Humoral Immune Response

Mediated by soluble proteins

called antibodies

Produced by activated B-

lymphocytes

Plasma cells

Antibodies are secreted into

circulation and mucosal fluids

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Antibodies

Neutralise and eliminate microbes & microbial toxins in

blood

lumens of mucosal organs

Prevent microbes present on mucosal surfaces/blood from colonising host cells/organs

Prevent establishment of infection

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Antibodies

Do not have access to microbes

that live and divide inside

infected cells

Intracellular microbes

Require cell-mediated immune

response to eliminate these

microbes

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Cell-Mediated Immune

Response

Defense against intracellular

microbes

Mediated by T-lymphocytes

2 main types:

1. Activating phagocytes to

destroy and ingested microbes

2. Directly kill infected host cells

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CONSEQUENCES OF IMMUNITY

DESIRABLE UNDESIRABLE

1. Natural resistance

against pathogens

2. Recover from

infection

3. Acquire immunity

to infectious

agents

1. Allergy &

Hypersensitivity

2. Autoimmune

diseases

3. Rejection of

organ

transplants

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1 Introduction to Immune System

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