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T Cells and Immunity. Antigen Recognition T cells only recognize antigens that are bound to glycoproteins in plasma membranes . T Cells and Immunity. MHC Proteins The membrane glycoproteins that bind to antigens Genetically coded in chromosome 6 The major histocompatibility complex (MHC)

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t cells and immunity
T Cells and Immunity
  • Antigen Recognition
    • T cells only recognize antigens that are bound to glycoproteins in plasma membranes
t cells and immunity1
T Cells and Immunity
  • MHC Proteins
    • The membrane glycoproteins that bind to antigens
    • Genetically coded in chromosome 6
      • The major histocompatibility complex (MHC)
      • Differs among individuals
t cells and immunity2
T Cells and Immunity
  • Two Classes of MHC Proteins
    • Class I
      • Found in membranes of all nucleated cells
    • Class II
      • Found in membranes of antigen-presenting cells (APCs)
      • Found in lymphocytes
t cells and immunity3
T Cells and Immunity
  • Class I MHC Proteins
    • 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
t cells and immunity4
T Cells and Immunity
  • Class II MHC Proteins
    • Antigenic fragments
      • From antigenic processing of pathogens
      • Bind to Class II proteins
      • Inserted in plasma membrane to stimulate T cells
t cells and immunity5
T Cells and Immunity

Figure 20–16 Antigens and MHC Proteins.

t cells and immunity6
T Cells and Immunity

Figure 20–16 Antigens and MHC Proteins.

t cells and immunity7
T Cells and Immunity
  • Antigen-Presenting Cells (APCs)
    • Responsible for activating T cells against foreign cells and proteins
t cells and immunity8
T Cells and Immunity
  • Phagocytic APCs
    • Free and fixed macrophages
      • In connective tissues
    • Kupffer cells
      • Of the liver
    • Microglia
      • In the CNS
t cells and immunity9
T Cells and Immunity
  • Non-phagocytic (pinocytic) APCs
    • Langerhans cells
      • In the skin
    • Dendritic cells
      • In lymph nodes and spleen
t cells and immunity10
T Cells and Immunity
  • 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
t cells and immunity11
T Cells and Immunity
  • 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
t cells and immunity12
T Cells and Immunity
  • CD4 Markers
    • Found on cytotoxic T cells and suppressor T cells
    • Respond to antigens on Class I MHC proteins
  • CD8 Markers
    • Found on helper T cells
    • Respond to antigens on Class II MHC proteins
t cells and immunity13
T Cells and Immunity
  • CD8 or CD4 Markers
    • Bind to CD3 receptor complex
    • Prepare cell for activation
t cells and immunity14
T Cells and Immunity
  • Costimulation
    • For T cell to be activated, it must be costimulated
      • By binding to stimulating cell at second site
      • Which confirms the first signal
t cells and immunity15
T Cells and Immunity
  • Two 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
t cells and immunity16
T Cells and Immunity
  • Cytotoxic T Cells
    • Also called killer T cells
    • Seek out and immediately destroy target cells
t cells and immunity17
T Cells and Immunity

Actions of Cytotoxic T Cells

  • Release perforin:
    • To destroy antigenic plasma membrane
  • Secrete poisonous lymphotoxin:
    • To destroy target cell
  • Activate genes in target cell:
    • That cause cell to die
t cells and immunity18
T Cells and Immunity

Figure 20–17 Antigen Recognition by and Activation of Cytotoxic T Cells.

t cells and immunity19
T Cells and Immunity
  • Slow Response
    • Can take up to 2 days from time of first exposure to an antigen for cytotoxic T cells to reach effective levels
t cells and immunity20
T Cells and Immunity
  • Memory TC Cells
    • Produced with cytotoxic T cells
    • Stay in circulation
    • Immediately form cytotoxic T cells if same antigen appears again
t cells and immunity21
T Cells and Immunity
  • Suppressor T Cells
    • Secrete suppression factors
    • Inhibit responses of T and B cells
    • Act after initial immune response
    • Limit immune reaction to single stimulus
t cells and immunity22
T Cells and Immunity
  • Helper T Cells
    • Activated CD4 T cells divide into
      • Active helper T cells (TH cells):
        • secrete cytokines
      • Memory TH cells:
        • remain in reserve
t cells and immunity23
T Cells and Immunity

Figure 20–18 Antigen Recognition and Activation of Helper T Cells.

t cells and immunity24
T Cells and Immunity
  • Four Functions of Cytokines
    • Stimulate T cell divisions
      • Produce memory TH cells
      • Accelerate cytotoxic T cell maturation
    • Attract and stimulate macrophages
    • Attract and stimulate NK cells
    • Promote activation of B cells
t cells and immunity25
T Cells and Immunity

Figure 20–19 A Summary of the Pathways of T Cell Activation.

b cells and immunity
B Cells and Immunity
  • B Cells
    • Responsible for antibody-mediated immunity
    • Attack antigens by producing specific antibodies
    • Millions of populations, each with different antibody molecules
b cells and immunity1
B Cells and Immunity
  • B Cell Sensitization
    • Corresponding antigens in interstitial fluids bind to B cell receptors
    • B cell prepares for activation
    • Preparation process is sensitization
b cells and immunity2
B Cells and Immunity
  • B Cell Sensitization
    • During sensitization, antigens are
      • Taken into the B cell
      • Processed
      • Reappear on surface, bound to Class II MHC protein
b cells and immunity3
B Cells and Immunity

Figure 20–20 The Sensitization and Activation of B Cells.

b cells and immunity4
B Cells and Immunity
  • 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 cells and immunity5
B Cells and Immunity
  • B Cell Division
    • Activated B cell divides into
      • Plasma cells
      • Memory B cells
b cells and immunity6
B Cells and Immunity
  • Plasma Cells
    • Synthesize and secrete antibodies into interstitial fluid
  • Memory B Cells
    • Like memory T cells, remain in reserve to respond to next infection
b cells and immunity7
B Cells and Immunity
  • Antibody Structure
    • Two parallel pairs of polypeptide chains
      • One pair of heavy chains
      • One pair of light chains
    • Each chain contains
      • Constant segments
      • Variable segments
b cells and immunity8
B Cells and Immunity
  • Five Heavy-Chain Constant Segments
    • Determine five types of antibodies
      • IgG
      • IgE
      • IgD
      • IgM
      • IgA
b cells and immunity9
B Cells and Immunity
  • Variable Segments of Light and Heavy Chains
    • Determine specificity of antibody molecule
b cells and immunity10
B Cells and Immunity
  • Binding Sites
    • Free tips of two variable segments
      • Form antigen binding sites of antibody molecule
      • Which bind to antigenic determinant sites of antigen molecule
  • Antigen-Antibody Complex
    • An antibody bound to an antigen
b cells and immunity13
B Cells and Immunity
  • A Complete Antigen
    • Has two antigenic determinant sites
    • Binds to both antigen-binding sites of variable segments of antibody
  • B Cell Sensitization
    • Exposure to a complete antigen leads to
      • B cell sensitization
      • Immune response
b cells and immunity14
B Cells and Immunity
  • Hapten (also called partial antigen)
    • Must attach to a carrier molecule to act as a complete antigen
  • Dangers of Haptens
    • Antibodies produced will attack both hapten and carrier molecule
    • If carrier is “normal”
      • Antibody attacks normal cells
      • For example, penicillin allergy
b cells and immunity15
B Cells and Immunity
  • Five Classes of Antibodies
    • Also called immunoglobulins (Igs)
    • Are found in body fluids
    • Are determined by constant segments
    • Have no effect on antibody specificity
b cells and immunity16
B Cells and Immunity

Figure 20–21a–b Antibody Structure and Function.

b cells and immunity17
B Cells and Immunity

Figure 20–21c–d Antibody Structure and Function.

b cells and immunity18
B Cells and Immunity
  • Seven Functions of Antigen-Antibody Complexes
    • Neutralization of antigen-binding sites
    • Precipitation and agglutination: formation of immune complex
    • Activation of complement
    • Attraction of phagocytes
    • Opsonization: increasing phagocyte efficiency
    • Stimulation of inflammation
    • Prevention of bacterial and viral adhesion

Figure 22–22

b cells and immunity19
B Cells and Immunity
  • Primary and Secondary Responses to Antigen Exposure
    • Occur in both cell-mediated and antibody-mediated immunity

Figure 22–22

b cells and immunity20
B Cells and Immunity
  • Primary and Secondary Responses to Antigen Exposure
    • First exposure
      • Produces initial primary response
    • Next exposure
      • Triggers secondary response
      • More extensive and prolonged
      • Memory cells already primed
b cells and immunity21
B Cells and Immunity
  • The Primary Response
    • Takes time to develop
    • Antigens activate B cells
    • Plasma cells differentiate
    • Antibody titer (level) slowly rises
b cells and immunity22
B Cells and Immunity
  • The Primary Response
    • Peak response
      • Can take 2 weeks to develop
      • Declines rapidly
    • IgM
      • Is produced faster than IgG
      • Is less effective
b cells and immunity23
B Cells and Immunity
  • The Secondary Response
    • Activates memory B cells
      • At lower antigen concentrations than original B cells
      • Secretes antibodies in massive quantities