Cell Mediated Acquired Immunity

7/31/2019 Cell Mediated Acquired Immunity

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Cell-Mediated immuneresponse

Recognition and the binding of a maturelymphocyte to an antigen induces thelymphocyte to divide rapidly.

This proliferation of lymphocytes is calledclonal selection. Two types of clones areproduced: short-lived activated effector cellsand long-lived memory cells

These effector cells activate B cells orMacrophages, or destroys infected, tumor, ortransplanted cells directly or through release of

soluble mediators i.e. cytokines and Controls


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Why it is known as cellmediated

It is based on type of microbial invaders those are facedby immune system.

Microbial invaders fall into two broad categories-In one category-organisms that originate outside the

body including most bacteria, fungi as well as manyprotoza and invading helminths.The second category consists of the organisms that

originate or live inside the bodys own cells includingviruses and intracellular bacteria or protozoa.

Cell mediated immune system is directed againstintracellular or endogenous invaders that invade cells.


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How the antigen ispresented to lymphocytes

All nucleated cells of body trap endogenos antigenswith the help of MHC 1 receptors on their surface and

then present to cytotoxic T- lymphocytes also having

receptors for trapping.

Its the main way of cell mediated immunity.

Professional antigen presenting cells also help in


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Types of antigenpresenting cells (APC)

1. Dendritic cellsPhagocytic dendritic cells (DC) of tissues

(Most potent stimulators of T cell responses)

Germinal Center Dendritic Cells (GCDCs)Thymic Interdigitating Dendritic Cells (IDCs)

Interstitial Dendritic Cells

2. B cells as APCPresent Ag to T cells (TH2)

3. Macrophages


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The Role of the MHCIn infected cells, MHC molecules bind and transport

antigen fragments to the cell surface, a process

called antigen presentation.

T cell can then detect the antigen fragmentdisplayed on the cells surface.

Depending on their source, peptide antigens are

handled by different classes of MHC molecules,

mainly 2 are involved in the cell mediated immunity

which are as follows-


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The Two Classes Of MHCMolecules

MHC Class I

Class I MHC molecules are found on almost all

nucleated cells of the body and Presents Agderived from intracellular source/cytosol.

They display peptide antigens to cytotoxic T

cells

cytotoxic T cells kill target


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MHC Class II

Class II MHC molecules are located mainly on

dendritic cells, macrophages, and B cells and

Present Ag derived from extracellular or cell-bound

source.

These antigen-presenting cells then displayantigens to inflammatory T cells and helper T cells.


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T Cells

Originate in stem cells in bone marrow

Change to T cells in thymus

Become sensitized to specific antigenProduce cell-mediated immunity following T cell

activation (in peripheral lymphoid organ) -

Cytotoxic T cells (Tcyt) Kill infected targetcells Inflammatory (TH1) and Helper (TH2) T

cells Activation of other cells (macrophages

and B cells, respectively)


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Way of functioning of T-cells

Inflammatory T cells = TH1

Activate macrophages

MHC class II-restricted

Helper T cells (TH2)

Help to eradicate extracellular pathogens

Activated helper T cells secrete cytokines that

stimulate other lymphocytes and Provide


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Cytotoxic T cells = Tcytv Recognize virus-infected cellsv Kill infected cells directly by inducing

apoptosis (programmed cell death cellsuicide)

v MHC class I-restricted

Natural Killer Cells

Recognize virus-infected cells and cancer cells

They directly release lethal chemicals, leading to

apoptosis


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Helper T cellsParticipate in both the Humoral and Cell-Mediated Immune Responses

The HIV virus attacks Helper T cells

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T- cell receptorEach T cell receptor consists of two different

polypeptide chains.

The tips of the chain form a variable (V) region; the

rest is a constant (C) region.

T cells can bind to an antigen that is free or on the

surface of a pathogen

A single T cell has about 100,000 identical antigen

receptors.

Fig 43 9


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Fig. 43-9

Anti gen-

binding

site

Antigen-

bindingsite

Antigen-

binding

site

Disulf ide

bridge

Variabl

eregions

Constant

regions

Transmembrane

region

Plasma

membra

ne

Light

chain

Heavychains T c

ell

ch

ain

ch

ain

DisulfidebridgeCytoplasm of T cell

(b) T cellreceptor

Cytoplasm of B cell(a) B cell

receptor

B cell

V V

C C

V V

C C C C

VV


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Steps of the Cell MediatedResponse

1. Infected cell presents antigen on ClassI MHC

2. Cytotoxic T Cell recognizes antigenand binds to it

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Steps of the Cell MediatedResponse

3 . Cytotoxic T Cell reproduces (clonesitself)

4. Cytotoxic T Cells bind to antigen boundto infected cells and release perforin

to lyse infected cells

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Response to InfectedCells

Cytotoxic T cells are the effector cells in cell-mediatedimmune response.

Cytotoxic T cells make CD8 , a surface protein that

greatly enhances interaction between a target cell and

a cytotoxic T cell.

Binding to a class I MHC complex on an infected cell

activates a cytotoxic T cell and makes it an active killer.

The activated cytotoxic T cell secretes proteins that

destroy the infected target cell.

Fig 43-17


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Fig. 43 17

Antigen-

presenting

cell

Peptideantigen

Cell-mediated

immunity(attack

oninfected

cells)

Class II MHC

moleculeC D4

TCR (T cellreceptor)Helper T

cellHumoral

immunity(secretio

n of antibodies by

plasmacells

)

Cytotoxic Tcell

Cytokines

Bcell

Bacterium

+

+ +

+


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Memory T-cells


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Overall diagramatic presentation of cell mediated immunity


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Primary versus SecondaryResponse

Primary Response = initial immune

response to antigen -

The first exposure to a specific antigen represents theprimary immune response

During this time, effector B cells called plasma cells

are generated, and T cells are activated to theireffector forms

Secondary Response = immune response when-


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Primary vs. SecondaryResponse

The secondary response is much faster than theprimary response

More antibody & T cell production in a shorteramount of time

Primary

response

Secondary Response

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Cell Mediated Acquired Immunity