Intro to Ab and B lymphocytes Flashcards
What is the role of the immune system?
Responsible for recognition and elimination of pathogens
What are the 2 parts of the immune system?
Innate - rapid and non-specific
Adaptive - takes longer because must recognise specific antigens
BUT has ability to generate memory
What are the 2 parts of the adaptive immune system?
Cell mediated immunity - mediated by T - cells
Humoral immunity - Ab mediated
What are Ab produced by?
B lymphocytes
How do B lymphocytes generally work?
Bind to Ag and aid clearance of pathogen by multiple mechanisms
Is the specificity of Ab for B cells the same?
Each B cell differs in specificity for Ab
10^8 different types of B cell .˙. 10^8 Ab specificities
What happens upon proliferation of B cells?
B cells form clones of identical cells, each w/ individual specificity for Ag
What is the difference between a BCR and Ab?
BCR = surface bound Ag Ab = secreted form BCR
What is the structure of an Ab?
4 polypeptide chains
2 identical heavy chains = 50kDa
2 identical light chains - 25kDa
Held together by disulphide bridges
Glycosylated
What is the Ab binding site made up of?
Made up of all 4 chains = variable region
Where is the Ab glycosylated?
Highly glycosylated at the conserved regions w/in heavy chain
Post-translational mod - addition of sugars to amino acids
Important role in determining Ab effector
What superfamily is the Ab part of?
Ig superfamily
Other members include - TCR, MHC, Ab receptor
What is the basic structure of the Ig superfamily?
Each 110aa segment of Ab forms discrete, compactly folded protein domain
Domains stabilised by multiple non-covalent interactions and disulphide bridges
Each protein domain made up of beta sheets (significant aa homology) - form loops of 60-70aa = immunoglobulin fold
Proteins joined by polypeptide chain = hinge region
What are the domains of the Ab?
Constant domains make up majority of structure of Ab
Variable regions make up domain at end of Ab - Ag binding site
What does the Ab structure allow?
Allows for stable structure and multiplicity of functional variants
Flexibility for Ag binding
What are the 2 parts that an Ab can be split into?
Can be split by proteases at the hinge region into 2 identical Fab fragments
F(ab)2 - Ag binding region
And Fc region - fragment crystallisable
Comprises only of constant region of heavy chain
What does the Fab region determine?
Specificity
Affinity
Avidity
of interaction w/ Ag
What does the Fc region confirm?
Confirms functional properties of Ab
As can be recognised by Fc receptors on immune cells + can bind to complement to trigger complement cascade
What is the main role of an Ab?
To bind to Ag - can happen in multiple different ways
How can the Ab bind to Ag? (immune complexes)
- multiple ab bind to Ag causes formation of immune complexes
C1q part of complement cascade - can bind to immune complexes and leads to cascade of events
eg. pore formation in pathogen membrane and death of pathogen - immune complexes can bind to Fc receptors on innate immune cells
Most Fc receptors have low affinity for single Ab and req. immune complexes to bind and trigger intracellular signalling
eg. opsonisation/phagocytosis
How can Ab bind to Ag? (ACMC)
Ab can bind to Ag on surface of infected cells
recognised by Fc receptors/effector cells eg. NKC
Aids in recognition and killing - Ab cell mediated cytotoxicity
How can Ab bind to Ag? (activation neutrophils and mast cells)
Binding of Ab to Ag
Then recognised by Fc receptors which causes activation neutrophils and mast cells
Enables sensitisation and ability to release preformed mediators
eg. IgE to enhance allergic sensitisation
How many Ab classes are there?
5 classes
9 subclasses
How are Ab classes defined?
Defined by heavy chain constant region
also determines effector function
Different classes are adapted to function in different parts of body
List the classes of Ab
IgG - 4 subclasses IgM IgA - 2 subclasses IgD IgE
What is the molecular weight of an Ig molecule?
150kDa
IgM/IgA - larger as can form multi-meric structure
Are the serum concentration of the Ab the same?
Differ between classes
Serum levels IgG much higher - 1 (IgG3) - 9 (IgG1) mg/ml
IgE lowest - 0.00005 mg/ml
Which Ab classes have a longer half life?
IgE & IgM
Able to persist for longer - these important for secondary immune responses
Which Ab class was the first to evolve?
IgM
major component in innate immune response
What is the heavy chain of IgM encoded by?
u gene w/ 4 Ch domains
What does it form when found as monomers?
Forms BCR on most B cells
in association with Iga and Igb chains
a - alpha
b - beta
What percentage of the serum Ig does IgM contribute to?
10% serum Ig
Describe the affinity of IgM
Typically low affinity receptor but when secreted form pentameric structure
Held together by disulphide bonds
Describe the pentameric structure of an IgM
5 IgM associate together and stabilised by a J-chain
Increases avidity Ab
What is the function of an IgM Ab?
Activate complement - (classical)
Describe the properties of the IgM Ab
Mean serum conc - 1.5 mg/ml
Molecular wt - 970 kDa
1/2 life - 10 days
Describe IgG Ab
Monomeric Ab
Heavy chain gamma gene
3 heavy chain conservative domains - 3Ch domains
What percentage of serum Ig doe IgG make up?
70-75%
Major circulating Ab
How many subclasses of IgG are there and what are they?
4 sub-classes
G1,2,3,4 (decreasing abundance)
Subclasses have different affinities to Fc receptors and have unique biological function
What role do IgG Ab have during pregnancy?
IgG can cross the placenta and protect the foetus
What is the main function of IgG Ab?
Have major role in activation of complement (classical) and opsonin (FcR)
Describe IgA Ab
Heavy chain alpha gene and 3Ch domains
15-20% circulating Ig
How many subclasses of IgA are there?
2
IgA - in serum as monomers
IgA2 - in mucous as dimers
Describe dimeric IgAs
2 IgA molecules associated w/ J chains
Found in secretion eg. tears, milk, saliva, sweat or epithelia line intestines and respiratory tract
What role do IgAs play?
Major role in 1st line of defence
Protection external surfaces
Can get localised mucosal response - different to systemic response
Does not activate complement - by classical pathway
What prevents IgA enzymes from being broken down by protease?
Secretory component added during transport through the epithelium
Protects from degradation from proteolysis
Aids release into mucous
How are IgA’s secreted?
- IgA dimers bind poly-Ig receptor on basolateral surface epithelial cells
- Complex endocytosed
- Carried through cytoplasm - vesicles fuse with luminal surface
- Ab released through proteolytic cleavage of poly-Ig receptor
- Dimer released attached to secretory component
Able to bind to musins in mucous and prevent cleavage
Describe IgD Ab
Heavy chain coded by delta gene
Monomer
3Ch domains
Where are IgD Ab most likely to be expressed?
On surface B cells w/ monomeric IgM
.˙. <1% serum Ab
What is the function of IgD Ab?
Have specific binding activity but no effector function
Instead involved in Ag triggered B cell differentiation
What are IgD Ab sensitive to?
Proteolytic degradation and heat
Describe IgE
heavy chain encoded by epsilon (E)
4 Ch domains - like IgM
How abundant are IgE levels in the serum?
Found at trace levels
Majority IgE found bound to mast cells and basophils through high affinity FCER1 receptors
Elevated in allergic or heavy parasite infection
What is the main function of IgE Ab?
Key to allergic response and parasitic infections
W/ Ag binding triggering degranulation mast cells
Very effective at killing multicellular organisms
Why are anaphylactic responses so quick?
As on first exposure IgE produced - binds to mast cells
hence, on subsequent exposure, IgE is waiting and ready on mast cells
What type of Ig is often expressed as a BCR on a B cell?
Often monomeric IgM/IgD on surface
Both have same Ag specificity on an individual B cell
What is the frequency of other types of Ig being expressed as a BCR?
10% Circulating B cells express IgG, IgA, IgE
Some tissue bias eg. mucosal B cells express IgA
What is the purpose of the intracytoplasmic tail?
Short tail anchors the surface bound BCR to membrane
Why do BCRs need to associate with a accessory molecule?
As the intracytoplasmic tail is short Accessory molecule aids intracellular signalling Ig alpha - CD79a Ig beta - CD79b carry out signalling fn of BCR
What are ITAM molecules and how are they phosphorylated?
Immunoreceptor Tyrosine based activation motifs
Ligation BCR causes phosphorylation of ITAM
What is the effect of ITAM phosphorylation?
Downstream effects - differentiation into plasma cells and Ab production
What is the effect of BCR ligation?
Clonal expansion
Following Ag recognition by BCR
B cells activated to proliferate
What is clonal expansion?
When one B cell gets activated it becomes a clone
What occurs after clonal expansion and describe the properties of these cells?
Majority of cells become effector cells/ plasma cells
Endoplasmic reticulum - plasma cells have multiple as it’s site of Ab production
Mitochondria - have lots of as require lots of energy to produce Ab
Describe the lifespan of plasma cells
Limited lifespan - die within few days (apoptosis)
Small proportion cells remain as long lived memory B cells - able to respond rapidly upon reinfection
What triggers class switching?
Triggered by Ag interaction w/ B cells
What is the primary Ab response?
IgM
What occurs upon activation? (secondary response)
B cell changes Ab production to other classes Ab - esp IgG
Can also be IgA, IgE - depends on functional requirement
What can occur during class switching?
Somatic hypermutation can occur
Point mutation into variable region gene sequence
Produced closely related clones with slight difference Ag specificity and affinity
What occurs during somatic hypermutation?
Mutations that improve Ag affinity are selected for and expanded in affinity maturation
Requires T cell help
How can Ab be used in clinic?
Ab can be raised against any organic substance
Can do this by injecting Ag into animals (mice) - produce Ab against Ag
How can Ab be produced for clinical use? (more detail)
Take Ab producing B cells from mice and fuse w/ B cell of tumour line eg. myeloma cells
Produces hybridoma cells which can undergo continuous proliferation
Hybridoma cells screened for desired Ab clone
Clones are grown and produce large amount identical Ab - can be harvested for use
What are the issues with Ab produced from mice?
Constant region heterogeneity = Mice Ab have different constant region and so recognised by human immune system as foreign
Illicits prod. of anti - antibodies -> formation of immune complexes + inflammation
How were issues with mice Ab overcome?
Engineering Ab
What are the different types of engineered Ab?
Chimeric Ab - made by fusing murine variable region (responsible for Ag binding) w/ human Fc constant domains
= less immunogenic Ab and Fc region can interact w/ human effector cells to generate immune response
Humanised Ab - murine hyper-variable region w/ fully human Ab
Even less immunogenic but still possibility as hyper-variable region foreign
Fully human monoclonal Ab - phage display libraries (in vitro McAb) or humanised mice
How can the different types of engineered Ab be differentiated?
Type of Ab identified by suffix that’s used
Chimeric - iximab
eg. infliximab - treats rheumatoid arthritis, Crohn’s disease and ulcerative colitis
inhibits TNF - alpha
Humanised - zumab
eg. alemtuzumab - treats B cell leukaemia
targets Ag CD52 on T + B lymphocytes
Fully human Ab - umab
eg. adalimumab - treats rheumatoid arthritis, Crohn’s disease and ulcerative colitis
inhibits TNF - alpha
What are checkpoint inhibitors?
New class monoclonal Ab Interfere w/ inhibitory signals that regulate lymphocytes
eg. Ipilimumab - anti - CTLA4
Nivolumab - anti - PDI
Revolutionised cancer therapy
Blocking above checkpoints = T cells can become activated + prod. responses against tumours
