The Immune System

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presenting cell

Memory

Helper T cells

Helper T cell

Cytotoxic T cell

Memory

Cytotoxic T cells

Active

Cytotoxic T cells

Antigen (2nd exposure)

Secreted

antibodies

Defend against extracellular pathogens by binding to antigens,

thereby neutralizing pathogens or making them better targets

for phagocytes and complement proteins.

Defend against intracellular pathogens

and cancer by binding to and lysing the

infected cells or cancer cells.

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Slide 42

Acquired Immune Response

Key

Stimulates

Gives rise to

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Memory

Helper T cells

Antigen-

presenting cell

Helper T cell

Engulfed by

Antigen (1st exposure)

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+

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Defend against extracellular pathogens

Memory

B cells

Antigen (2nd exposure)

Plasma cells

B cell

Secreted

antibodies

Humoral (antibody-mediated) immune response

Slide 43

Acquired Immune Response

Cell-mediated immune response

Defend against intracellular pathogens

Active

Cytotoxic T cells

Memory

Cytotoxic T cells

Memory

Helper T cells

Antigen-

presenting cell

Antigen (2nd exposure)

Helper T cell

Engulfed by

Antigen (1st exposure)

Cytotoxic T cell

Key

Stimulates

Gives rise to

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Slide 44

Helper T Cells: Respond to Nearly All Antigens

A surface protein called CD4 binds the class II MHC molecule.

This binding keeps the helper T cell joined to the antigen-presenting cell while activation occurs.

Activated helper T cells secrete cytokines that stimulate other lymphocytes.

Positive Feedback in the Immune System enhances the process until some endpoint or maximum rate is reached.

Slide 45

The central role of helper T cells in humoral and cell-mediated immune responses

Antigen-

presenting

cell

Peptide antigen

Cell-mediated

immunity

= attack on

infected cells.

Class II MHC molecule

CD4

TCR (T cell receptor)

Helper T cell

Humoral

immunity

= secretion of

antibodies by

plasma cells.

Cytotoxic T cell

Cytokines

Positive Feedback …

B cell

Bacterium

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Slide 46

Cytotoxic T Cells: A Response to Infected Cells

Cytotoxic T cells are the effector cells in cell-mediated immune response.

Cytotoxic T cells make CD8, a surface protein that greatly enhances interaction between a target cell and a cytotoxic T cell.