W11L16 - T & B Cell Activation

Question 1

Stages of Acquired Immunity

Answer 1

1. Establishment of Infection
2. Induction of Adaptive Response
3. Adaptive Immune Response
4. Immunological Memory

Question 2

Functions of the Adaptive Immune Response

Answer 2

Eliminates foreign substances and abnormal body cells
Cleans dead cells
Magnifies and shuts down inflammatory responses
Activates complement
Regulates itself

Question 3

Humoral (B cells) & Cellular (T cells) Immunity Activation

Answer 3

Humoral
- deal with extracellular microbes
- anything surrounding B cell can be recognised
Cellular
- deal with extracellular and intracellular
- cant see pathogen without APC
- processed antigen displayed on MHC Class 1 on APC and presented to T cell

Question 4

What happens at each of the stages of adaptive immunity?

Answer 4

1. Establishment of Infection
   - APC presents antigen to naive T cell
   - antigen recognised by B cell
   - has to reach threshold
2. Induction of Adaptive Response
   - clonal expansion of B and T cells
   - differentiation of B and T cells into specific cells e.g. helper T cells
3. Adaptive Immune Response
   - elimination of antigens
   - apoptosis
4. Immunological Memory
   - surviving memory cells spread throughout the body

Question 5

Adaptive Effector Responses

Answer 5

B cells
- antibodies
T cells
1. CD4+ helper cells 
- Th1 - help CD8+ via cytokines
- Th2 - help B cells via cytokines
- Th3, Th17, Th100
2. CD8+ cytotoxic T cells - destroy infected and altered cells

Question 6

T Cell Activation

Answer 6

Initiated by interactions between APCs and T cells
APCs process antigen into small fragments (epitopes)
Epitopes must be:
- inside MHC complex molecules
- on the APC cell surface
- presented to the TCR on T cells
MHC Class 1 - present intracellular antigens to CD8+ cytotoxic T cells only
MHC Class 2 - present extracellular antigens to CD4+ T cells only

Question 7

Where is a T cell first activated or primed?

Answer 7

Lymph nodes

Dendritic cells pick up antigen, mature, and migrate to lymph nodes

Question 8

How are APCs activated?

Answer 8

Pattern recognition receptors detect pathogens
Toll-like receptors recognise specific pathogen structures
Causes up regulation of co-stimulatory molecules

Question 9

Two Signal Model of T cell Activation

Answer 9

Signal 1: Antigen recognition
- ensures that response is Ag-specific
Signal 2: Costimulatory molecules on APCs
- B7 family = CD80 &CD86
- these bind CD28 on T cells
CD40 = binds CD40L on T cells
Signal 1 + Signal 2 = full activation

Question 10

What is the initial response of T cells to activation?

Answer 10

Makes IL-2 which is required for itself (autocrine)
Resting T cells express moderate affinity IL-2 receptors
Activated T cells express high affinity IL-2 receptors
Binding of IL-2 to receptors signals T cell to enter cell cycle
IL-2 induces T cell proliferation

Question 11

Steps of T Cell Activation - Summing Up

Answer 11

1. Pathogen activates APC
2. APC expresses co-stimulatory molecules
3. Pathogen taken up e.g. by phagocytosis
4. Pathogen processed
5. APC presents peptide in MHC to naive T cells
6. T cell activated
7. T cells leave lymph node

Question 12

B Cell Activation

Answer 12

B cells respond to epitopes in intact antigens
Via B cells receptor
BCR = antibody on B cell surface
Steps:
1. Antigen bound by B cell surface receptor
2. Antigen internalised and degraded to peptide fragments
3. Fragments bind to MHC class 2 and are transported to cell surface

Question 13

Two Signals for B Cell Activation

Answer 13

Signal 1
- B cells binds to peptide
- B cell presents peptide in MHC class 2 molecules to CD4+ T cells
Signal 2 
- cytokines Il-4 and IL-5
- secreted by activated CD4+ T cells
Cytokines further activate B cells