B cells and antibodies Flashcards
(29 cards)
What do B cells look like?
- B cells have membrane bound immunoglobulin (antibody) on their surface
- If immunoglobulin is attached to the surface is can be called a B cell receptor
- They also have a very large nucleus
What do B cells differentiate to become
B cells differentiate and go through clonal expansion upon activation to become plasma cells; these no longer have membrane–bound immunoglobulin but secrete soluble immunoglobulin into the blood and extracellular space
Where do B cells come from?
How many B cells do you make every day?
- B cells are ‘born’ in the bone marrow
- B cells are descended from HSC (haematopoietic stem cells→lymphoid progenitor)
- You make one billion B cells every day
What do B cells produce and how?
- Immature B cells in the bone marrow randomly select gene segments coding for the two proteins making the B cell receptor (BCR = membrane-bound immunoglobulin)
- The BCR is made up of two identical heavy chains (Hc) and two identical light chains (Lc)
- Each chain is encoded by genes assembled from multiple segments
- Antibodies do not obey the one gene = one protein rule
- Once committed to making a particular antibody it will be all it can make
From the diagram:
The red portion is the antigen binding site
constant region determines the isotype of antibody (function)
What is the function of Fab regions?
What is the function of Fc regions?
- The light and heavy chains come together on the top part of the Y and that determines antigen binding
- Antigen binding region (Fab) allows antibody to recognise different peptides in a larger protein
- The constant region (Fc) determines the function of the antibody
- All the different constant regions have receptors on different cells of the immune system and thats helps mediate their function
How does immature B cell DNA become mature B cell DNA?
- One complete heavy chain is made up of four gene segments pasted together: one V, one D, one J and one C
- Chromosome 14 contains multiple, slightly different copies of each gene segment
- Each copy of chromosome 14 randomly selects one D and one J segment, then it picks one V segment – joins this to DJ
- Then it adds on a constant region(CM)
- approx 50 variable regions
- approx 10 variable constant regions (5 different isotypes)
- regions not selected are chopped out of the region and excised
Explain B cell gene rearrangement
What happens if the first allele of the heavy chain fails?
What happens if the second allele of the heavy chain also fails?
When the B cell is going through development, it tries to make a heavy chain first, it uses a placeholder for the light chain to see if the heavy chain works first
It goes through functional rearrangement- choosing one gene segment from each of the sections to make a heavy protein which is tested to see if the heavy chain protein binds to any antigen
If it does, it keeps that heavy chain and progresses on to rearranging the light chain (similar to heavy chain but without the constant region at the bottom)
If the first allele of the heavy chain isnt successfully spliced together, it will try a different allele but if the second function arrangement also fails and doesnt bind to any antigen, the B cell will be sent to undergo appoptosis
Explain B cell gene rearrangement
Functional/productive rearrangement means that a particular combination ‘works’
- The heavy chain is paired with a surrogate light chain (SLC) to form a pre-BCR
- When this happens, light chain processing can occur
Why do B cells generate a massive diversity of receptors?
- Every mature B cell produces one (and only one) kind of BCR made up of Light chains and Heavy chains
- V(D)J recombination is a very costly process that must be (and is) strictly regulated and controlled
- This mix and match strategy is very expensive (around 90% of B cells don’t make it out of the bone marrow),
- But it means that we have a massive diversity of receptors on our B cell population
- B cells can recognise ANYTHING (107– 1011 different specificities!)
- Overall, this is a simple scheme that really works well
- This rearrangement system is also used for the T cell receptor (TCR)
How does B cell activation occur?
- We need to get from the B cell (with Ig stuck on its membrane) to a plasma cell (making soluble, protective antibodies)
- Every B cell BCR recognises a different, specific antigen
- ‘cognate’ antigen
- The BCR actually binds a tiny part of the cognate antigen
- called the EPITOPE
- When the B cell recognises its epitope, it needs to send a signal to the nucleus
- This switches on the genes involved in B cell activation
- Activation = differentiation to a plasma cell
Describe BCR signalling
- To generate a signal, many BCR need to be brought together on the B cell surface - ‘clustering’
- This can happen when the epitope is a repeating sequence
- Or when many antigens are found close together
- Most bacterial and viral surfaces are made of repeating proteins
How does co-operation complement activate adaptive immunity?
How is opsonization involved?
Co-operation complement activates adaptive immunity
- Complement is an opsonin – tags invaders
- B cells have complement receptors (CR
- If an antigen is opsonised, the B cell can bind it via BCR-antigen and via complement receptor-complement interactions – more efficient, less antigen needed
- This helps to bring BCR and Complement receptor (CR) together on the B cell surface – more clustering so the signal is amplified
How does Innate immunity direct adaptive immunity?
- Cross-linking of BCR and CR amplifies the signal
- CR is therefore a co-receptor
- This is most important when antigen concentration is low (start of infection)
- This is one of the links between innate and adaptive immune responses
- The innate immune response decides that an invader is dangerous (tags it with complement) then recognition of the danger by the B cells (surface CR) brings the adaptive immune system into action
- It’s the innate immune system that decides what’s dangerous
B cells must be activated before they can make antibodies:
Are all B cells naïve before activation?
- Most B cells are naïve – they have never seen their cognate antigen
- Some B cells are experienced – they have seen their cognate antigen
- Naïve and experienced B cells need to be activated to produce antibodies but the activation pathways are different.
- The threshold for activation of an experienced B cell is lower
What is clonal selection?
Coupling activation (antigen recognition) to proliferation is the basis for clonal selection. Only selcting B cells that will be useful for particular infections
- During development the progenitor cell to a large number of lymphocytes, each with different specificity
- Interaction with its cognate antigen during an infection selects the B cell as special
- Only activated B cells can respond to IL-2 and proliferate
- This forms a clone of B cells – all with identical BCR (proliferation and differentiation of pathogen-activated lyphocytes give rise to effector cells that terminate the infection)
Do activated B cells express the same proteins as naïve B cells?
- Activated B cells express different proteins to naïve B cells
IL-2R binds IL-2, promotes B cell proliferation
- Activated B cells are now ready to mature:
- Class switching- depending on the pathogen, a different antibody is generated
- Affinity maturation- The B cell can refine the antibody that it produces but its cant make a new one
- Career choice (memory or plasma cell)
Class switching: Immature B cells express what two classes of soluble antibody on their surface?
What antibody is primarily produced in the early stages of infection?
- sIgM and sIgD:
- (same heavy chain mRNA, different splicing)
- IgD in circulation - who knows?
- sIgD on naïve B cell - important for activation
- Naïve B cells leave the bone marrow to look for their cognate antigen
- Activated B cells make antibody - starting with IgM
- But they change this to make IgG, IgA or IgE
- Class switching
- Antibody class dictated by its tail (Fc region)
- To class switch, the B cell just cuts out the IgM segment and pastes in an IgG (or IgA or IgE) segment instead
What is Affinity maturation?
- Survival of the fittest for B cells
- B cells undergo SOMATIC HYPERMUTATION
- Point mutations of the V regions
- Test against antigen in the area
- better binding, B cell lives
- worse/same binding is bad news for that cell
- Only B cells making the highest affinity antibody (best defence) survive
How do B cells choose between becoming a plasma cell or memory cell?
- Plasma cell - antibody factory
B cell goes to bone marrow or spleen, produces secreted BCR (soluble antibody)
Go to lymph nodes
- Memory cell - not as dramatic, also move to the lymph nodes
But just as important
Are plasma cells more effective than memory cells?
Why is it harder to generate memory B cells in old age?*
Memory B cells are more effective
- High affinity BCR - respond to low levels of antigen
- Appropriate isotype of antibody, high affinity antibody
- Easier to activate
- Memory B cells are ‘ready to go’ in a second invasion
- They might be long lived, they might proliferate now and again to keep their numbers up, nobody’s sure
- But we do know that they can give you life-long immunity
It becomes more difficult to generate B cells as you get older because we tend to loose our lymphoid progenitor cells, imbalance with more myeloid cells and less lymphoid progeniotor cells
Adjuvants can be added to vaccines an extra boost
What are antibodies?
Antibodies are soluble effector molecules of adaptive immunity
What are the 5 classes (isotypes) of antibody?
Which is the first antibody made?
What is its function?
Five classes (isotypes) of antibody
- IgA, IgG, IgM, IgE, IgD (acronym- GAMED):
- Different locations and different functions
- IgM – first antibody made, usually (primary immune response)- pentamer with 10 antigen binding region
- IgA protects mucosal surfaces e.g respiratory tract, GI tract and genitounitary tract (interface between the body and the environment)
- IgE- immunity to parasites such as helminths like Schistosoma mansoni, Trichinella spiralis, and Fasciola hepatica
- IgD- to signal the B cells to be activated.
- IgG- main type of antibody found in blood and extracellular fluid,
- IgD, IgG and IgE are soluble
Explaint the properties of IgM
Explain Co-operation of innate immunity (complement) with adaptive immunity (IgM)
- Pentameric
- important in the early stages of an immune response
- Best at fixing comlement
- 5 constant regions and 10 antigen binding regions
- Good at activating complement
- Classical activation pathway
- Needs two molecules of C1 brought together
- IgM Fab bind their antigen
- IgM Fc can now bind C1
- IgM has 5 Fc, so it can bring several C1 together…
- Cascade activated
- Production of the C3 convertase
Co-operation of innate immunity (complement) with adaptive immunity (IgM)
Explain the properties of IgG
IgG has different isotypes (different Fc regions):
- gG1, IgG2, IgG3 and IgG4
- These different isotypes are good at:
- Neutralising viruses- cuts of viral transmission pathway
- Diffuses through tissues for added protection
- Protecting the fetus
- Maternal IgG crosses the placenta
- Babys adaptive immune system doesnt develop untill 6 months of age
- Giving passive immunity (IVIG)
- e.g. in hepatitis infection
