22.2

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IMMUNITY

22.0 IMMUNITY

22.1 : Immune response

22.2 : Development of immunity : primary and secondary response

Learning outcomes

Students should be able to:

a) Explain the primary and secondary immune responses.

b) Explain the concept of self and non-self.

Learning Outcomes : c) Explain humoral and cell mediated immune response

Immunity development

Lymphocytes originate from the

bone marrow

Some continue their maturation

in the bone marrow, → B cells

Some move to the thymus and

continue their maturation there,

→ T cells

B cells and T cells then circulate

throughout the blood and lymph

(concentrated in the spleen,

lymph nodes and other lymphatic

organs)


Immune Response

Immune Response

Humoral Cell mediated

Involve mainly B cell

Produce antibody in

plasma (body fluid ~

humor)

With the help of TH cell

Involve mainly T cell


There 2 main types of T cells:

i. cytotoxic T cells (TC)

- act to destroy cancer cells and cells

infected by pathogens

ii. helper T cells (TH or T4)

- act to help activating/stimulating TC and

B cells in immunity responses

B cells have antigen receptors which recognize and

bind to specific free antigens

Immune Response


Classes of T cell

Lymphocyte T

T4 / CD4 cell

T8 / CD8 cell

Helper T cell

(TH)

Cytotoxic T cell

(TC)

Suppressor T cell

(TS)


Immune Response

T cells have T cell receptors which recognize

and bind to antigens found on infected cells or

on any antigen-presenting cell (can be a

macrophage)

Major histocompatibility complex (MHC) of body

cells will present/display fragments of antigens

to be identified by the T cells


Role of TH in immunity

• Usually, a macrophage performs phagocytosis on

pathogens, and there will be antigen fragments in

the macrophage

• Receptor of TH recognizes and binds the antigen

fragment presented by MHC of the macrophage

• The macrophage secretes interleukin-1 (a type of

cytokine ~ that activates lymphocytes)

• This activates TH to divide, producing clones of TH

(all with receptors for that specific antigen)


• Activated TH will also secrete interleukin-2 that

activates TC (to perform cell-mediated immune

response) and B cells (to perform humoral immune

response)


Humoral immune response

Macrophage ingest & destroy

the antigen by phagocytosis

Fragment of antigen is

displayed on the cell surface

together with MHC

To form a MHC-antigen

complex

These cells is known as

Antigen Presenting Cells

(APC)



TH with a specific receptor bind to the complex

Macrophage secrete Interleukin-1 (IL-1)

Which activates TH to secrete Interleukin-2 (IL-2)

Which stimulate the activated TH to proliferate & differentiate

to form 2 clones:-

1. Activated TH clone

2. Memory TH clone



Proliferate :

Divide rapidly to form many TH which has

identical MHC-antigen receptor (clone)

Differentiate

Form different types of TH (activated TH &

memory TH)



Humoral immune response involves B cells

activation and production of antibodies that circulate

in the blood and lymph

This is initiated when B cells are activated by binding

with free antigens or by IL-2 secreted by activated TH

Activated B cells proliferate and differentiate to form

memory B cells and plasma cells

Memory B cells are cells that may respond rapidly

upon subsequent exposure to the same antigen

Plasma cells are cells that secrete antibody specific

to that antigen



[Memory B cells and plasma cells may divide to

form clones to enhance the immune response]



Antibodies secreted

by plasma cells,

released into blood,

lymph and other

extracellular fluid

Antibodies move to

the infected area;

antigen-antibody

interaction occurs


Cell mediated immune response

The immunity response which depends on the action of TC cells

This is initiated when TC is activated by binding to antigen fragment displayed by MHC of an infected cell / cancer cell

IL-2 secreted by activated TH enhances the activation of TC

Activated TC may proliferate and diffrentiate, forming more activated TC cells and memory T cells

Memory T cells may respond rapidly upon subsequent exposure to the same antigen



Activated TC discharges perforin molecules which create pores in the membrane of the infected cell

Water and ions flow into the infected cell; the cell lyses

Learning Outcomes : a) Explain the primary and secondary immune responses.

Types of immunity

Immunity

Passive Active

Natural Artificial Natural Artificial

Antibody from:

pregnant mom to fetus

Nursing mother to infant

Antibody from

Injected antibody from animal / people to another people

Exposed naturally to antigen

By immunization or vaccination


Development of Immunity


Development of Immunity

- For active immunity, it develops within 2 stages:-

Immunity

Primary immune response Secondary immune response

1st exposure to the antigen 2nd exposure to the same antigen


-Has a longer lag period (where no antibody is produced)

- Due to the time needed for the specific B cell to:

Primary Immune Response

Become activated

Proliferate (producing clone)

Differentiate (plasma cell & memory cell)

-Plasma cell secrete antibody (mainly Ig M)

- Amount of antibody is relatively low

- After a short time, amount of antibody decrease


- Has a very short lag period (due to the presence of memory B

cells ~ immunological memory)

- Production of antibody is :-

Secondary Immune Response

Faster

Higher Amount

More prolonged (remain longer)

- Antibody level tends to remain high for longer period

- Plasma cell secrete antibody (mainly Ig G)


Artificial Active Immunity

- Obtained by vaccination / immunization

- Obtained by injecting small amount of vaccine into the body

- Which trigger immune response

- Effect :- produce antibody against injected antigen

- Vaccine is inactivated toxins or weakened / dead pathogen

- Which trigger immune system to produce antibody,

but can no longer cause disease


Vaccination

• BCG (tuberculosis) • After birth / 13 yrs

• Hepatitis B • After birth / 1 mth / 6 mths

• Triple Antigen (DPT)

-Diphteria (sore throat)

-Pertusis (whooping cough)

-Tetanus

• 3 / 4 / 5 mths

• Polio • 3 / 4 / 5 mths

• MMR

-Measles

-Mumps

-Rubella

• 4 yrs

Learning Outcomes : b) Explain the concept of self and non-self.

Self and non-self recognition The immune system may recognize and distinguish a certain cell /

tissue / organ as self or non-self

It depends on the MHC on every cell surfaces (that will be recognized by T cell receptors)

A person will have a same set of MHC molecules

The sets of MHC are different for any 2 individuals (except for

identical twins)

MHC is a self marker unique to each individual

If the MHC shows antigen fragments that are considered foreign, immune response occurs

Normally, the MHC and antigens occurring naturally in own body are not regarded as foreign and will not trigger the immune system


The organs of donors and recipients are

screened for compatibility (to reduce

rejection)

Only organs from donors with compatible

MHC are used

If the organ is non-compatible, the MHC will

be recognized as foreign(non-self)

The transplanted organ is destroyed by

T-lymphocytes (Tc cells)

Self and Non-self :Organ Transplant


- The transplanted organ is rejected by the recipient

body

- Immunoresponsive agents/drugs are used to

prevent rejection of transplanted organ

- Immunoresponsive agents/drugs suppress the immune

system

-Example of immunoresponsive agents is cyclosporine

- another method to prevent rejection is by using radiation

such as X-rays at lymph nodes

- To destroy lymphocytes.

Self and Non-self :Organ Transplant

22.2

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