7. T cell activation 1&2

Question 1

What do B and T cells recognise?

Answer 1

Antibodies bind to native protein antigen

• to be recognised by a T-cell receptor the protein antigen must first be processed into peptide antigens
• then the peptide antigens must be bound to MHC molecules and only then can the antigen be presented to the T cell receptor
• B cells recognise native proteins whereas, T cells recognise degraded proteins bound by MHC molecules

Question 2

What is MHC restriction?

Answer 2

T cell recognise peptide antigen only when it is bound to a host body’s own MHC molecules
Normally, T cells are stimulated only in the presence of self MHC + antigen
T cells develop tolerance to self MHC in the thymus

Question 3

What are CD4 and CD8 co-receptors?

Answer 3

CD8 co-receptor binds the 𝛂3 domain of MHC class heavy chain so MHC class I molecules present peptides only to CD8T cells 
CD4 co-receptor binds to the 𝛃2 domain of the MHC class II molecules ensuring that peptides bound to MHC class II only stimulate CD4T cells

Question 4

What is the CD3 complex needed for?

Answer 4

Expression of the T cell receptor at the T cell surface requires association with proteins of the CD3 complex - these proteins transmit signals into the T cell when the T cell receptor binds antigen

Question 5

What are the two signals required for T cell activation?

Answer 5

CD28 molecule on T cells delivers a co-stimulatory signal upon engaging either of its ligands
- B7.1(CD80) or B7.2(CD86)

Question 6

What is the importance of interleukin-2?

Answer 6

Binding of IL-2 to its receptor signals T cell to enter the cell cycle - inducing T cell proliferation

Question 7

What is different about an effector T cells response to infection?

Answer 7

Don’t depend on co-stimulatory signals

1. Stimulation of naive T cell (recognition)
2. Proliferating T cell (proliferation/differentiation)
3. Active effector T cells recognise and kill virus infected target cells

Question 8

What the function of a T cell?

Answer 8

Necessary for cell mediated immunity

- actively destroy infected cells

Question 9

How are CD8 T cells recognised Ag in association with MHC class I?

Answer 9

1. Virus infects cell
2. Viral proteins synthesised in cytoplasm
3. Peptide fragments of viral proteins bound by MHC class I in ER
4. Bound peptides transported by MHC class I to the cell surface
5. Cytotoxic T cell recognises complex of viral protein with MHC class I and kills infected cell

Question 10

What are cytotoxic T cells?

Answer 10

CD8 T cells have a cytotoxic function, known as cytotoxic T cells
Effector T cells kill cells infected with viruses or other intracellular pathogen 
- only recognise cells bearing its corresponding peptide antigen presented by a MHC class I molecule 
- has a binding site for MHC class I molecules: to determine strict specificity

Question 11

What is the function of a CD8+ T cell?

Answer 11

CD8 + CTL kills target cells that display antigenic fragments of cytotoxic pathogens (usually pathogens) bound to MHC class I molecules on cell surface

Question 12

What is the action of perforin on the target cell?

Answer 12

Aids in delivering contents of granules into the cytoplasm of target cell

Question 13

What is the action of granzymes on the target cell?

Answer 13

Serine proteases, which activate apoptosis once in the cytoplasm of the target cell

Question 14

What is the action of granulysin on the target cell?

Answer 14

Has antimicrobial actions and can induce apoptosis

Question 15

What are the categories of viral immune evasion pathways, MHC class I?

Answer 15

1. Blocks peptide entry to endoplasmic reticulum
2. Retention of MHC class I in endoplasmic reticulum
3. Degradation of MHC class I
4. Binds MHC class I at cell surface

Question 16

What is the significance of CD4 cells being recognised with Ag in association with MHC class II: TH1?

Answer 16

1. Macrophage engulfs and degrades bacterium, producing peptides
2. Bacterial peptides bound by MHC class II in vesicles
3. Bound peptides transported by MHC class II to the cell surface
4. Helper t cell recognises complex of peptide antigen with MHC class II and activates macrophage

Question 17

What is the significance of CD4 cells being recognised with Ag in association with MHC class II: TH2?

Answer 17

1. Cell-surface immunoglobulin of a B cell binds bacteria: the cell engulfs and degrades them , producing peptides
2. Bacterial peptides bound by MHC class II to the cell surface
3. Bound peptides transported by MHC class II to the cell surface
4. Helper T cell recognises complex of peptide antigen with MHC class II and activates B cell

Question 18

How does the T helper (TH2 cell) produce plasma cells?

Answer 18

1. Antigen recognition induces expression of CD40 ligand and cytokines by the TH2 cell which activate the B cell
2. B-cell proliferation and differentiation to antibody-secreting plasma cells

Question 19

How do B cells produce antibodies against a virus and a bacterial component?

Answer 19

1. B cell binds to virus through viral coat protein
2. Virus particle is internalised and degraded
3. Peptides from internal proteins of the virus are presented to the t cell, activating the B cell
4. Activated B cell produces antibody against viral coat protein

Question 20

What cytokines are produced by the TH1 and the TH2 cells?

Answer 20

TH1 = IFN-𝛄, TNF-𝛂, IL-2, IL-12, IL-23
- cell mediated immunity 
- Pro-inflammatory cytokines
TH2 = IL-4, IL-5, IL-6, IL-10
- antibody mediated immunity 
- anti-inflammatory cytokines