PMI02-2026 Adaptive immunity: T cell response

Question 1

What 3 ways does apoptosis occur?

Answer 1

Secretion of TNF which binds to the TNF receptor
The cytotoxic T cell secretes perforin and granzymes
FAS ligands binds to FAS

Question 2

How many chains does an MHC class II receptor have?

Answer 2

2

Question 3

What do the 2 chains MHC class II form?

Answer 3

an antigen binding domain

Question 4

How does the structure of MHC class I compare to that of MHC class II?

Answer 4

The overall structure is similar but the alpha chain is larger in MHC class I and forms a complete peptide binding chain 
Only the alpha chain is anchored in the membrane of MHC class I

Question 5

How does the Th bind to MHC class II?

Answer 5

Th binds via TCR to MHC II complex
TCR of the Th is associated with CD4 which binds to MHC class II

Question 6

What is the purpose of CD3 when the T cell binds to the MHC antigen complex?

Answer 6

CD3 transmits the activating signal to the T cell following

Question 7

What protein do cytotoxic T cells express?

Answer 7

CD8

Question 8

What MHC class molecules does CD8 bind to?

Answer 8

Class I

Question 9

What cells express MHC class I molecules?

Answer 9

All nucleated cells

Question 10

What cells express MHC class II molecules?

Answer 10

Only cells involved in immune response: dendritic cells, macrophages, activated B cells and T cells

Question 11

What is antigen processing?

Answer 11

The process that breaks down proteins to peptides which may be bound by MHC molecules and exported to the cell surface

Question 12

How does MHC I antigen processing occur?

Answer 12

The antigens come from the cell or come from phagosomes
Cytosomic proteasomes are then responsible to break down this protein into peptides
These peptides enter the ER via the TAP proteins
MHC I molecules is synthesised and the peptide binds to the alpha chain of this molecule
It is then presented on the surface of the nucleated cell so it can come into contact with the CD8+ Tc cell

Question 13

How does MHC II antigen processing occur?

Answer 13

Antigens are endocytosed in membrane vesicles
It is internalised by APCs and enter lysosomes where is it degraded by  lysozymes, that break proteins into peptides
MHC class II is synthesised in the ER and the invariant chain initially blocks the binding site to prevent the proteins from binding other proteins 
The invariant chain is also involved in transporting the MHC class II molecule to a vesicle
As the two vesicles fuse, the invariant chain breaks down leaving a class-II invariant chain peptide
The peptide binding site now becomes available for binding to peptides 
MHC class II with extracellular peptides are transported to the surface of the APC, where they may be recognised by antigen specific T helper cells

Question 14

What are naive T lymphocytes?

Answer 14

Those yet to encounter their cognate antigen

Question 15

What are the 3 signals required for Th cell activation?

Answer 15

When the T cell receptor binds to the antigen/ MHC complex, CD4 binds to MHCII/ antigen and a protein kinase enzyme in the T cell phosphorylates CD3, which in turn recruits further kinases that transmits further signals to the T cell
CD80/CD86 binds to CD28. This is needed to stimulate production of IL-2, which further activates the T cell
Cytokines stimulate T cell proliferation and differentiation into effector cells and memory cells

Question 16

What are the 3 signals required for Tc cell activation?

Answer 16

When the T cell receptor binds to antigen MHC, CD8 binds to MHC I/antigen and a protein kinase enzyme in the T cell that phosphorylates CD3, which in turn recruits further kinases that transmits further signals to the T cell
CD80/CD86 binds to CD28, which is needed to stimulate production of IL-2 which further activates the T cell
Th cells produce IL-2 which is needed to CDL proliferation. Th cells further stimulates via inflammatory ligands such as: CD40 which binds to CD154 on the Tc
Once activated, Tc proliferate to form effector and memory cells

Question 17

How does the immune response reduce once the infection is cleared?

Answer 17

T cells change from expressing stimulating receptors (CD4, CD8) are replaced with inhibitory receptors (CTLA4, LAG3) that don’t activate target cells

Question 18

Where does positive selection of T cells occur?

Answer 18

In the thymus cortex

Question 19

What occurs during positive selection?

Answer 19

T cells that do not bind self MHC undergo apoptosis
T cells that bind to self MHC II in the thymus cortex become CD4 single positive cells
T cells that bind to MHC I maintain expression of CD8 but lose expression of CD4

Question 20

Why does positive selection occur?

Answer 20

To enrich T cells that bind to MHC

Question 21

What receptors do immature T cells express?

Answer 21

TCR as well as Cd4 and CD8 (double positive cells)

Question 22

Why does negative selection of T cells occur?

Answer 22

To remove T cells that recognise self-antigens complexed with MHC

Question 23

What occurs to SP T cells after positive selection?

Answer 23

SP T cells that have survived positive selection migrate to the medulla
SP T cells that bind weakly to self-antigens associated with MHC, survive
SP T cells that bind strongly to self- antigen/MHC undergo apoptosis or become regulatory T cells

Question 24

What is the purpose of regulatory T cells?

Answer 24

inhibit or down regulate immune response