Explain how B cells are formed.
B lymphocytes come from the Bone.
- Intermediate stages to the formation of B cells - pro-B and pre-B cells.
- This differentiation pathway allows the B cell to acquire specific foreign receptors.
- When an immune response occurs, important to recognise that these mature B lymphocytes can differentiate even further.
They can mature into:
- Memory B cells
- Plasma Cells
Memory B lymphocytes allow better response to second infection & vaccination induced immune response.
- It's an unusual molecule as it can be on surface of cell but can also be secreted into blood plasma.
What is an antibody (immunoglobulin)
- They function as B lyphocyte receptor for antigen.
- Soluble recognition molecules secreted by B lymphocytes.
- High capacity for recognising fine molecular detail.
- Important functions:
1) Directly kill or neutralise microbes and/or their toxins
2) Help target phagocytes to microbes.
3) Regulation of immune responses (indirectly).
4) Clearance of antigens and termination of immune response.
5) Mediate allergic responses.
6) Cause immune complex diseases.
7) Mediate autoimmune cell destruction.
Describe the basic structure of an IgG molecule
- Antibody molecule secreted by B cell is similar to surface immunoglobin except that it does not have a transmembrane region.
- Heavy chain has 3 domains
- Light chain has 2 domains
- Antibody has 2 binding sites giving it better avidity.
Hinge of antibody (bit between light and heavy chains) gives it flexibility.
How do we make antibodies of infinitely different specificities?
We encode an antibody molecule by combining different genetic segments together.
1. germline diversity of genes = lots of different genes that can encode different parts of an AB molecule
2. Combinatorial diversity = diversity in the way we combine them together. (SOMATIC RECOMBINATION).
3. Junctional Diversity = we change the type of 'glue' we use to stick the segments together, so the glue also changes the structure of the protein (somatic recombination)
Give an overview of somatic recombination:
Think of the genes at the top being laid out in the line
How would you make the light pink one bind with the yellow one?
You can loop out bits of DNA and recombine with other points
How do you get different sub-classes of antibodies?
- Antibodies can maintain specificity but change their Fc (constant) region of the heavy chain.
- The heavy chains encoded by different numbers (alpha, delta etc) can give rise to certain sub classes of antibody.
Think of different antibodies in the class as different toolkits.
Fc region therefore gives you the function of the antibody.
Describe the structure of IgM and IgA
- IgM molecule has a 5x avidity ot an IgG molecule because of its pentameric structure.
- IgA has dimeric structure with a secretory piece. Secretory piece allows transport of antibody across mucosa into mucosal lumen but also protects them against enzymatic degradation.
Why is IgM good at activating complement?
- Because C1q binds to one bound IgM molecule to cause activation.
- But C1q has to bind to at least two IgG molecules
- Binding of C1q to Ig activates C1r which cleaves and activates the serine protease C1s.
Where can these antibody subclasses be found in the body?
Which antibody can be transported across the placenta/
Clonal Selection of B cells allows specificity in the immune response.
How does this happen?
- Undergoes somatic recombination and has B lymphocytes which recognise different receptors.
- We get infected by red antigen which is detected by B cells but not the other B cells.
- The other coloured cells continue to circulate and eventually die and be replaced by other coloured cells.
- But the red B cell here then undergoes extensive proliferation and differentiation.
- Each B cell produced from the red B cell is a clone of the original
- For a few days we can have many thousands of cells.
When all the B cells are cloned, they may need to change their function but not their specificity.
How does this work/what does this mean?
So B cells can say 'I arrived as an IgG molecule but i need to change my Fc regions and become a different Ig (whether IgA or IgM)
- This process is called class switching
- Class switching is accomplished by recognition of molecules called cytokines.
Cytokines are produced by components of the innate immune system and T lymphocytes.
Which complement pathway do antibodies activate? (and what are the steps of this pathway)
Antibody fixes complement to activate the classical pathway.
C1qrs --> C4 --> C4b --> C4b2 --> C4b2b --C3 --C3b
Many receptor cells have receptor for Fc portion of antibody molecule (phagocytic cell recognises Fc portion of antibody).
What are the Fc receptors on phagocytes?
Cellular recognition via FcR is important for effector function. How does this binding process work?
1. Antibody binding to bacterium.
2. Antibody-coated bacterium binds to Fc receptors on cell surface.
3. Macrophage membrane surrounds bacterium.
4. Antibody binds antigens on the surface of target cells.
5. Fc receptors on NK cells recognise bound antibody.
6. Crosslinking of Fc receptors signals the NK cell to kill the target cell.
What is required (generally) for the process of B cell production?
Most B cell responses required help from CD4+ T cells.
How does this process work?
1. Helper T cell adheres to the B cell and begins to synthesise IL-4 and CD40 ligand.
2. The helper T cell reorients its cytoskeleton and secretory apparatus towards the B cell.
3. IL-4 is released into and is confined to the space between the B cell and the T cell.
Where do T cells come from?
1. They are derived from bone marrow hematopoietic stem cells that migrate to the thymus.
2. 98% of T cells die in the thymus by positive and negative selection.
3. Only T cells with TCR able to recognise foreign peptides bound by MHC escape to the periphery.
So they come from bone marrow and live in the thymus.
Explain the formation of T cells, starting in the thymus and ending in circulation.
1. In the thymus, T-cell progenitors give rise to billions of thymocytes each with a different T cell receptor.
2. Thymocytes are positively selected by epithelial cells in the cortex of the thalamus.
3. Positively selected thymocytes survive and divide.
4. Positively selected thymocyte clones are negatively selected in the thymic medulla.
5. Clones surviving negative selection leave the thymus for the circulation.
Where are thymocytes negatively selected?
In the thymic medulla
Where are thymocytes positively selected?
- In the cortex of the thymus
How do T lymphocytes recognise foreign material?
- They have a receptor made up of 2 polypeptide chains and there are two domains in each of the polypeptide chains.
T cells recognise different forms of antigen to B cells. How so?
- B cells use Ab to recognise conformational epitopes or tertiary structure.
Conformational epitopes can be made up of discontinuous stretches of AA's.
T cells use TCR to recognise 'linear epitopes' - enzymatically digested out of antigens via antigen processing.
What are MHC molecules?
They are very specialised peptide transporters.
The main function of MHC is to bind to antigens and display them on the cell surface for recognition by the appropriate T cells.
Describe the process by which MHC I would present its antigens to the T cells
1. Virus infects cell.
2. Viral proteins synthesised in cytoplasm.
3. Peptide fragments of viral proteins bound by MHC class I in endoplasmic reticulum
4. Bound peptides transported by MHC Class 1 to cell surface.
5. Cytotoxic T cell recognises complex of viral peptide with MHC class I and kills infected cell.
Explain how MHC II molecules work
1. Macrophages engulfs and degrades bacterium, producing peptides.
2. Bacterial peptides bound by MHC Class II in vesicles.
3. Bacterial peptides transported by MHC Class II to cell surface.
4. Th1 cell recognises complex of peptide antigen with MHC class II and activates macrophage.
5. Cell-surface immunoglobulin of B cell binds bacteria and engulfs and degrades them, producing peptides.
6. Bacterial peptides bound by MHC Class II in vesicles
7. Bound peptides transported by the MHC Class II to the cell surface.
8. Th2 cell recognises complex of peptide antigen with MHC class II and activates B cell.
Compare the structures of MHC Class I & II
Class I: HLA-A, HLA-B, HLA-C
one MHC-encoded heavy (alpha) chain + beta-2 microglobulin
Class II: HLA-DR, HLA-DP, HLA-DQ
one MHC encoded alpha chain + one MHC encoded B chain
Where would you find MHC I molecules expressed?
- expressed on nucleated cells
- Carry peptides generated in the cytosol or ER
- Present peptides to CD8+ T cells
- Surveillance for virus infection and altered self.
Where would you find MHC II molecules expressed?
- Expressed on specialised antigen presenting cells (normally!)
- Carry peptides generated in the endosomal compartments
- Present peptides to CD4+ T cells
- Surveillance for exogenous pathogens.
Explain how T-cell activation required both antigen and co-stimulatory signals.
How do immunosupressive drugs such as cyclosporin and tacrolimus operate?
How are dendritic cells able to turn on T cell responses?
Specialised dendritic cells are required to turn on T cell responses
- They can capture and process antigen
- They can migrate from peripheral tissues
- They have MHC Class II molecules
- They can provide 'costimulation' - a second signal required to activate T cells.
Dendritic cells can also sense the environment
- They have a range of PRR, including TLRs
- They make a range of cytokines to influence T cell differentiation
What is clonal expansion?
Clonal expansion is an essential feature of acquired immune response
What are the 4 main types of CD4+ T cells?
What is the basis of acquired immunity?
Memory to previous exposure.
Where does antigen presentation happen?
How do B cells migrate to the primary follice?
Through entering lymphoid organs through high endothelial venules and migrating to the primary follice.