Adaptive Immune System

Question 1

Antigen presenting cells

Answer 1

1. Dendritic cells (activate B and T cells)
2. Macrophages (mostly T cells)
3. B cells (to activate T cells)

Dendritic cells store antigen on surface to present to B cells
All APCs (including DCs) break antigen into peptides and present on MHC for T Cell activation

Question 2

Notes on signals for lymphocyte activation

Answer 2

Need two signals to activate
1. via antigen specific receptor
- BCR for B cells recognising intact antigen
- TCR for T cells recognising antigen peptide fragment on MHC

2. via co-stimulatory molecules
   - For T cells: expressed by APCs in response to engagement of PRRs
   - For B cells: expressed by T cells in response to activation by APCs

Outcome = clonal expansion of cell with identical receptors

Question 3

Burnet theory re clonal selection of lymphocytes

Answer 3

• Population of lymphocytes, each one able to recognise a particular antigen
• On exposure to that antigen, the cell(s) able to recognise it divide rapidly -> giving rise to a clone of lymphocytes of identical specificity -> able to destroy invading pathogen

Question 4

Notes on B cells

Answer 4

• Generated in the BM
• Main role = antibody production (immunoglobulins)
• Immunoglobulins = globulins (i.e. non-albumin proteins) that have an immune function.
• Each B cell expresses unique surface receptor specific for a particular foreign antigen - this receptor is identical to the antibody it will make (called surface membrane immunoglobulin)
• Activated B cells divide and differentiate into plasma cells
• Plasma cells make antibodies identical to the surface receptor

Question 5

Notes on antibodies

Answer 5

Soluble proteins
Exquisite ability to bind target with high affinity
**Basic structure:
- **2 identical heacy chains
- 2 identical light chains
**2 functional regions
- **2 identical variable regions - binds target
- 2 identical constant regions - effector functions

Five types of antibody classes with different functions
Fight pathogens by:
- Neutralising target - block viral entry into cell, block effect of toxins
- Facilitate uptake by macrophages - which express Fc receptors, opsonisation
- Activating complement - also helps with opsonisation

Question 6

Notes on T cells

Answer 6

• Each T cells expresses unique surface receptor specific for a particular foreign antigen - TCR (rigid in structure, differs to antibody).
• Two chains - variable and constant regions. The two variable regions come together to crease the antigen binding site.
• TCR recognises antigen which has been broken into peptides and bount to presentation molecules (called MHC; HLA in the human system). *Contrasts with B cells and antibodies which recognise intact antigen

**T cell as effectors
**T cells develop in thymus from precursors generated in the bone marrow
Two types - CD4 and CD8. Both express surface T cell receptor

**T cell activation
- **Antigenic peptide (+MHC) binds to TCR (signal 1)
- Co-stimulation signal received (signal 2)
- T cell activated to:
- 1. Divide
- 2. Differentiate into effector cells - cytotoxic T cells (kill virally infected cells), helper T cells (secrete cytokines which govern type of immune response), regulatory T cells (control extent of immune response)
- 3. Become memory T cells - long lasting, respond faster on repeat exposure to antigen

TCR remains bound to T cell. *Unlike B cells where antibody is released after activation

Question 7

Notes on MHC molecules

Answer 7

Two types (HLA in humans) molecules
1. MHC Class 1
- expressed by most cells
- Present peptides derived from the breakdown of proteins in the cytoplasm
- Recognised by CD8 T cells (Cytotoxic T lymhpcytes)
2. MHC Class 2
- expressed only by APCs
- Present peptides derived from engulfed antigen
- Recognised by CD4 T cells (helper cells)