3. Adaptive Immunity: B-cell Response Flashcards
LOs
· Describe the structures of antibodies and understand the functions of the antigen binding (Fab) and constant (Fc) regions.
· Describe how B cell antigen receptor diversity occurs.
· Discuss tissue distribution and effector functions of the different classes (isotypes) of antibodies.
· Describe the role of helper T cells in activating and controlling the B cell response and immunoglobulin class switching
Functions of B-cells
I
- B- cell receptor features
- Signalling capability?
- Signalling dependent on?
1
- B cell receptor comprises an
antibody molecule of:
~ 2 identical heavy chains
~ 2 identical light chains
2
- antibody molecule has a
transmembrane domain BUT no
independent signalling capability
3
B cell receptor signalling is dependent on Iga + Igb
a = alpha
b = beta
How do antibodies recognise antigens and bind?
- Antibodies bind to the surface of antigens
- Binding occurs via the tips of the Y shapes that are highly variable between B cells
- Binding can be physical such as lock and key, or
physicochemical such as hydrophilic/ or
electrostatic attractions - A single antigen can have multiple epitopes on its
surface that can be recognized by antibodies
expressed by, or secreted by different B cells
Antibody structure?
- comprise two identical heavy chains
- two identical light chains
- heavy chains are joined together by
disulphide bonds (identified by the dotted oval) - light chains are joined to the heavy
chains by disulphide bonds (identified by
dotted circles) - ovals that are joined together to form the
heavy and light chains structures are called
domains
Antibodies
↳ hinge
↳ Fab’
↳ Fc
What are the 5 antibody (immunoglobulin) isotopes/ classes?
IgG
IgM
IgD
IgA
IgE
(GAMED)
- IgG subclasses?
- How they differ?
- What determines the numbers/ which is the most abundant
- In humans 4 subclasses
- IgG1
- IgG2
- IgG3
- IgG4
2.
Differ in their constant region sequence
↳ encoded by diff gene segments
3.
IgG subclasses are numbered according to their abundance in human blood
IgG1 = most abundant subclass
- IgA subclasses?
- How they differ?
- Which subtype is where in the body?
1.
2 subclasses in humans
- IgA1
- IgA2
- IgA1 + IgA2 subclasses can be
dimeric or monomeric - so total = 4 different types of IgA
2.
Differ in their constant region sequences
↳ are encoded by diff gene segments
3.
- mostly IgA1 in blood
- in gut can be either IgA1 or IgA2
IgM and IgA can be polymeric
- what is IgM in blood? (number of chains)
- how are the IgM chains held together?
- What is IgA? (number of chains)
- IgM in blood is pentomeric
- comprises of 5 units of IgM
- irrelevant when IgM on cell surface as part of B-cell receptor (as that involves single units IgM)
- only when secreted is pentomeric struc relevant
2.
- 5 units held together by molecule
called J chain
- ‘J’ = ‘joining’
- IgA when secreted = monomeric OR dimeric
- dimeric = units held together by J chain
function of antibodies?
- Blocking
- Complement fixation
- Opsonisation
- ADCC (antibody dependent cellular cytotoxicity)
- Agglutination
- Mast Cell Degranulation
- 1st function of antibodies?
- EG?
1.
BLOCKING
Antibodies can function purely by binding to
target to prevent it binding to receptor
2.
- blocking toxin from binding to toxin
receptor
- blocking a virus preventing it from binding to its receptor
- 2nd function of antibodies?
- what happens?
1.
COMPLEMENT FIXATION
2.
- IgG + IgM can ‘fix’ complement when they bind
to a surface
- initiates the complement cascade
- results in production of proinflammatory molecules + formation of membrane attack complex
EXTRA
- antibodies are free in the serum
- when free in serum, do not initiate complement complement fixation
- in serum polymeric IgM molecules = planar (flat) formation
- when IgM binds to cell surface of pathogen, alters from being flat to spider shape = staple form
- when IgM = staple form on top of target, C1q identifies this change + can bind to IgM
- C1q binds to IgM + initiates cascade
- IgG is same (needs to be rigid structure bound to cell surface to initiate cascade)
- 3rd function of antibodies?
- what happens?
1.
OPSONISATION
2.
- The binding of IgG to the surface of a pathogen can make the pathogen more visible to phagocytes such as macrophages and neutrophils
- The bound IgG can be recognized by Fc gamma receptors of phagocytes
This facilitates phagocytosis - antibody can be referred to as an opsonin
- antibody coated target is said to be opsonized
- process is called opsonization
- Complement C3b can also be opsonin
- 4th function of antibodies?
- what happens?
1.
ADCC
antibody dependent cellular cytotoxicity
2.
- IgG antibody bound to a target cell
- can be recognized by Natural Killer (NK)
cells via their Fcg receptors
- NK cells with cross-linked Fc
receptors can secrete cytotoxic
granules into the synapse, resulting in
the death of the target cell via apoptosis
- 5th function of antibodies?
- what happens?
1.
AGGLUTINATION
(IgA = good agglutinator as has 4 binding arms)
2.
- Polymeric antibodies with multiple binding arms
can cross link pathogen resulting in formation of a
lattice
- This can ‘disarm’ pathogen
- Polyclonal IgA antibodies can be actively secreted
at mucosal sites such as gut, eyes + mouth - are at relatively high conc in mucus,
saliva + tears compared to other antibodies - IgA antibodies are transported across epithelia by
secretory component that binds to the J chain - IgA has a specific role in regulating microbial
populations due at least in part to its ability to
coat and agglutinate bacteria
- 6th function of antibodies?
- what happens?
- resting vs activated mass cell
- MAST CELL DEGRANULATION
2.
- Mast cells contain granules each containing
proinflammatory molecules including histamine
- Mast cells have receptors for IgE (Fce receptors)
- IgE can bind to Fce receptors of mast cells and
so long as the IgE is not cross linked, there will be
no consequences - If IgE bound to Fce receptors of mast cells
becomes cross linked, the mast cell will become
activated and granules released resulting in a
characteristic ‘allergic’ reaction
3.
- what is the antibody variable region?
- what does it consist of?
- what are segments assembled from?
- tip of antibody Y-shape = variable region
- variable region of heavy
chain = comprised of 3 segments
- Variable (V)
- Diversity (D)
- Joining (J) - segments that comprise each variable region are assembled from multiple alternatives during B
cell development
Illustration of immunoglobulin heavy and light chain loci
Illustration of immunoglobulin heavy and light chain loci
Immunoglobulin variable region genes before
commitment to become a B cell comprise multiple
alternatives for each of
- V (Variable)
- D (Diversity)
- J (Joining)
segments
EXTRA
(add more - 20-23 minutes???)
HEAVY CHAIN
- also has constant region = has gene segment for each of diff constant regions that antibody may use (relevant later in life for B-cell)
- once b cell development starts (commit to become B cells), loci start to change
- to develop unique receptors
assembly of unique B cell receptors
(add more 23-25???)
- single gene segments from multiple alternatives in the germline assemble to form a unique receptor sequence
Maths behind BCR variability
- numbers here don’t matter BUT maths behind it matters
B cell responses can be T-dependent or T-independent
what do they depend on/ require?
- T-dependent B-cell responses require the antigen to have a protein component
- T-independent B-cell responses depend on antigens having repeat subunit structures so that they can cross-link the B cell receptor
The germinal centre response
- sequence of events that result in affinity maturation (generation of high affinity antibodies)
- Centroblasts divide rapidly + undergo somatic
hypermutation of antibody variable region genes - They have no antibody on their cell surface
- Centroblasts mature into relatively non dividing
centrocytes and re-express antibody - Centrocytes will sample antigen stored by STROMAL cells called follicular dendritic cells IF their antibody has sufficient affinity
- Centrocytes able to sample antigen, process and present it to T follicular helper cells and acquire help to survive
- Centrocytes with low affinity antibody cannot compete for antigen, do not receive a T cell survival signal and die
- A Darwinian process of survival of the fittest
- Affinity maturation
Germinal centre response generates memory cells and plasma cells
- Memory B cells can reside in tissues and circulate in blood
- Can be IgM+ or class switched
- Have antibody variable regions genes that are mutated by somatic hypermutation
- Plasma cells home to bone marrow or gut
