Hafiz Waqar Ahmed Baber

560-FBAS/BSBT/S14

Department of Biotechnology

Faculty of Basic and applied sciences

Cell mediated and Antibody mediated immunity

Antigens

Antigens have 2 characteristics Immunogenicity: Ability to

provoke immune response

Reactivity:Ability of antigen to react specifically with antibodies it provoked

Entire microbes may act as antigen

Typically, just certain small parts of large antigen molecule triggers response (epitope or antigenic determinant)

Antigen Processing

Cellular Immunity T cells attack foreign cells and diseased host cells; memory of Ag

Three classes of T cells

1. Cytotoxic T cells (Tc cells) carry out attack

2. Helper T cells: help promote Tc cell and B cell action and nonspecific defense mechanisms

3. Memory T cells: provide immunity from future exposure to antigen

MHC Proteins Membrane glycoproteins, bind to antigens,

encoded by chromosome 6, form MHC. 2 Classes of MHC Proteins Class I: found in membranes of all nucleated

cells Pick up small peptides in cell and carry them

to the surface: T cells ignore normal peptides abnormal peptides or viral proteins activate T

cells to destroy cell

Class II: found in membranes of antigen-presenting cells (APCs) found in lymphocytes Antigenic fragments inserted in cell membrane to stimulate T cells

Antigens and MHC Proteins

Figure 22–16a (Navigator)

Antigen-Presenting Cells (APCs) Responsible for activating T cells against foreign cells and proteins

Phagocytic APCs :

Free and fixed macrophages:

in connective tissues

Kupffer cells:

of the liver

Microglia:

in the CNS

Pinocytic APCs

Langerhans cells:

in the skin

Dendritic cells:

in lymph nodes and spleen

Figure 22–16b

Antigen Presentation

Mechanism of T cell recognition and activation

Antigen Recognition

Inactive T cell receptors:

recognize Class I or Class II MHC proteins

recognize a specific antigen

Binding occurs when MHC protein matches antigen

TC cell Recognition

Antigen presentation MHC-I proteins

found on nearly all nucleated body cells

display peptides produced by host cells

TC cell activation1. binding of cytotoxic T cells (CD8 cells) to abnormal

peptides on MHC-I and

2. costimulation via a cytokine

triggers clonal selection: clone of identical T cells against cells with same epitope

TH cell Recognition

Antigen presentation

role of MHC-II proteins found only on antigen

presenting cells display only foreign antigens stimulate helper T cells (CD4

cells)

CD Markers Also called cluster of differentiation markers:

in T cell membranes

molecular mechanism of antigen recognition

more than 70 types:

designated by an identifying number

CD3 Receptor Complex

Found in all T cells

CD8 Markers Found on cytotoxic T cells and suppressor T cells

Respond to antigens on Class I MHC proteins

CD4 Markers Found on helper T cells

Respond to antigens on Class II MHC proteins

CD8 or CD4 Markers - Bind to CD3 receptor complex

Prepare cell for activation

Costimulation

For T cell to be activated, it must be costimulated:

by binding to stimulating cell at second site

which confirms the first signal

2 Classes of CD8 T Cells

Activated by exposure to antigens on MHC proteins:

one responds quickly:

producing cytotoxic T cells and memory T cells

the other responds slowly:

producing suppressor T cells

TH cell Activation

1. binding of helper T cells (CD4 cells) to epitope displayed on MHC-II of APC

2. costimulation via a cytokine

3. triggers clonal selection

TH cell Activation

Activation of Helper T Cells

Figure 22–18

Helper T Cells - Activated CD4 T cells divide into:

active helper T cells:

secrete cytokines

memory T cells:

remain in reserve

Attack Phase: Role of Helper T Cells Secretes

interleukins attract neutrophils,

NK cells, macrophages

stimulate phagocytosis

stimulate T and B cell mitosis and maturation

Coordinate humoral and cellular immunity

Figure 22–17 (Navigator)

Activation of Cytotoxic T Cells

Also called killer T cells Seek out and immediately destroy target cells

Actions of Cytotoxic T Cells

1. Release perforin:

to destroy antigenic cell membrane

2. Secrete poisonous lymphotoxin:

to destroy target cell

3. Activate genes in target cell:

that cause cell to die

Slow Response

Can take up to 2 days from time of first exposure to an antigen, for cytotoxic T cells to reach effective levels

Memory Tc Cells

Produced with cytotoxic T cells

Stay in circulation

Immediately form cytotoxic T cells:

if same antigen appears again

Suppressor T Cells

Secrete suppression factors

Inhibit responses of T and B cells

After initial immune response

Limit immune reaction to single stimulus

4 Functions of Cytokines

1. Stimulate T cell divisions:

produce memory T cells

accelerate cytotoxic T cell maturation

2. Attract and stimulate macrophages

3. Attract and stimulate NK cells

4. Promote activation of B cells

Figure 22–19

Pathways of T Cell Activation

Humoral Immunity Recognition

B cell receptors bind antigen, take in and digest antigen then display epitopes on its MHC-II protein

After costimulation by TH cell, divide repeatedly, differentiate into plasma cells, produce antibodies specific to that antigen

Attack antibodies bind to antigen, render it harmless, ‘tag it’ for destruction

Memory some B cells differentiate into memory cells

Humoral Immunity - Recognition21

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B Cells Responsible for antibody-mediated immunity

Attack antigens by producing specific antibodies

Millions of populations, each with different antibody molecules

Sensitization

Corresponding antigens in interstitial fluids bind to B cell receptors

B cell prepares for activation

Preparation process is sensitization

Figure 22–20 (Navigator)

B Cell Sensitization and ActivationDuring sensitization, antigens are taken into the B cell, processed, reappear on surface, bound to Class II MHC protein

Helper T Cells Sensitized B cell is prepared for activation, but needs helper T cell

activated by same antigen

B Cell Activation Helper T cell binds to MHC complex:

secretes cytokines that promote B cell activation and division

B Cell Division

Activated B cell divides into:

plasma cells -Synthesize and secrete antibodies into interstitial fluid

Memory B cells- Like memory T cells remain in reserve to respond to next infection

Any Question

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