PMI02-2005 Flashcards
What is the cell lineage of a B cell?
Pluripotent haematopoietic stem cell
Common lymphoid progenitor
Pre-B cell
Where do mature naive B cells migrate to after development in bone marrow?
Lymph nodes
Spleen
Where do B cells develop?
Bone marrow
List some functions of antibodies.
Inhibit attachment of pathogens to host tissue
Activate complement
Enhance phagocytosis
Induce degranulation of mast cells, basophils, eosinophils
Describe the basic structure of antibody.
Y-shaped
2 identical heavy chain polypeptides (50-60k MW) linked by disulphide bonds
Each heavy chain linked to one of 2 identical light chain polypeptides (25K MW)
Which part of the antibody binds to antigens?
Fab fragment
Which part of the antibody varies within a class?
Fab fragment
Which part of the antibody varies between classes?
Fc fragment
Describe the molecular structure of an antibody.
N terminal part of heavy chain forms 2 protein domains linked by a more flexible hinge region to 2 C terminal domains
Heavy N terminal domains fold with light chains to give 2 identical binding sides (Fab portions)
C terminal parts form Fc portion
Which of the C or N terminals mediates functions of antibodies such as complement activation?
C terminal (forms Fc portion)
What is an epitope?
Specific site on an antigen that binds to an antibody
What is an immune complex?
Antibody bound to antigen
May be a pair or a huge complex of many antibodies and antigens
What is the molecular part of the antibody that binds to an antigen?
3 loops (CDRs) between β-strands in the variable/Fab region
What are complementarity determining regions?
Loops between β-strands of the Fab region which bind to antigens
How are the CDRs numbered? Which often contributes most to binding affinity and specificity?
Numbered ascending from N (CDR1, CDR2, etc) to C terminal
CDR3 (also often the largest)
What are some effector functions of antibodies?
Activation of complement
Stimulate phagocytosis
Transport to mucosal surfaces to prevent invasion
Transfer across placenta
Induce degranulation
What role do antibodies play in complement activation?
Activate C1q when bound to antigen
Leads to lysis (MAC) or phagocytosis via complement of Fc receptors
How do immunoglobulin classes physically differ?
Number of Fc immunoglobulin domains
Whether or not they are monomeric or not
Which immunoglobulin class is most abundant in the serum? What about subclass?
IgG
IgG1 specifically (9mg/ml)
Which immunoglobulin class is most abundant at mucosal surfaces?
IgA
Which immunoglobulin can move across the placenta?
IgG
Describe the IgM class.
Pentameric
Each monomer has an additional immunoglobulin domain in the Fc region (4 in total)
Monomers linked by 2 disulphide bonds between CH3 and CH4 domains
Ring completed by a J chain linked by disulphide bonds
What is the J chain important for?
Oligomerisation and stabilisation of an altered conformation of CH4 domain to allow closer packing of monomers in IgM
Dimerisation of IgA
Which immunoglobulin classes are monomeric?
IgD
IgG
IgE
Which classes of antibody are produced by all newly activated B cells?
IgM
IgD
What allows the different classes of antibodies produced by one B cell to have the same specificity?
Identical VH and VC domains (form the Fab fragment)
What is IgA’s main function?
Prevent pathogen entry at mucosal surfaces
What is class-switching?
B cells change the class of antibody they make to tailor the response to the specific antigen/pathogen or anatomical location
Which classes of antibody have 4 constant domains (one extra than others)?
IgM
IgE
What is IgE usually associated with?
High affinity Fc receptors on surfaces of mast cells and eosinophils
What is the main effector function of IgM?
Strong activator of complement due to high affinity for C1q (5 binding sites!)
What are the main effector functions of IgG?
Activate complement cascade
Bind to phagocytic Fcγ receptors
How many subclasses are there of IgG?
4
Which IgG subclasses have higher affinities for Fc binding?
IgG1 and 3
What is the main effector function of IgA?
Bind Fcα receptors on phagocytes
Which Fc receptors do IgE bind to?
Fcε receptors
What does Fcγ receptor binding cause?
Th1-type response
Phagocytosis
Destruction
Describe the IgE-mediated Th2 response to parasites.
Cross-linking of IgE/Fcε receptor complexes on surface of cell by antigen triggers degranulation
Release of histamine, serotonin, proteases, cytokines, leukotrienes, etc
Causes recruitment and activation of lymphocytes, eosinophils, neutrophils and macrophages
On which chromosome is the gene for the immunoglobulin heavy chain?
Chromosome 14
Describe how diversity of antibodies is generated.
For Fab regions:
- heavy chain = 52VH, 27D, 6J segments )
- light chain = V and J segments
Random DNA recombination to produce functional exons
Random nucleotide additions at spliced sites
What is the possible diversity of antibodies?
> 10^14
Where are the gene loci for immunoglobulin light chains?
κ locus on chromosome 2
λ locus on chromosome 22
Describe B cell development in the bone marrow.
Negative selection of self-reacting B cells
Self-reacting B cells can undergo apoptosis or “receptor editing” (further DNA rearrangement of surface immunoglobulin VL region)
B cells that do not recognise self antigen or bind very weakly migrate to spleen and lymph nodes
What is the difference between class-switching and receptor editing?
Class-switching occurs after contact with antigen and involves changing the constant domains
OR
Receptor editing occurs during development in the bone marrow and involves changing the variable (light chain) domain
Where is IgD more abundant?
Upper respiratory mucosa
What do B cells recognise?
Antigens in solution
Immune complexes bound to Fc or complement receptors on macrophages/follicular dendritic cells
Describe the process of B cell activation.
- B cell receptor binds antigen to deliver signal 1
- Antigen and BCR is internalised and undergoes antigen processing. Presented on surface with MHC class II protein
- Th cell with same specificity recognises antigen-MHC complex and delivers signal 2 to B cell using CD154 to bind to CD40 (on B cell)
- Th cell secretes cytokines to stimulate B cell proliferation, differentiation and antibody production
What do plasma cells arising from B cells do?
Migrate to bone marrow and produce large amounts of antibodies
What do memory cells arising from B cells do?
Produce IgM and remain in circulation
How long to plasma cells arising from B cells last?
2-3 days
What does class-switching involve?
Further recombination of DNA with T cell help - looping out of CH exons, excision and religation of DNA
Then transcription and translation
Why is class-switching considered irreversible?
DNA is excised and religated permanently
Can only reverse if there are further CH regions downstream
Which immunoglobulins would be suited for a Th1 response?
Intracellular pathogens => IgG1 and 3 (complement activation, Fcγ receptor binding)
Which immunoglobulins would be suited for a Th2 response?
Extracellular parasites => IgE (degranulation)
Which immunoglobulins will be produced in the presence of IL-4?
IgE
IgG1
Which immunoglobulins will be produced in the presence of IL-21?
IgA
IgG3
Which immunoglobulins will not be produced in the presence of both IL-4 and IL-21?
IgA
Which enzyme initiates somatic hypermutation and class-switching?
Activation-induced cytidine deaminase (AID)
What does activation-induced cytidine deaminase do?
Initiates somatic hypermutation and class-switching
Converts cytosine to uracil to break base pairings and introduce DNA mutations
Describe the process of somatic hypermutation.
Requires antigen-specific T cell help
Activation-induced cytidine deaminase introduces point mutation in the V region exons during RNA transcription
Forms mutated antigen receptors which:
- have a higher affinity for antigen = preferentially selected
- do not bind to the antigen anymore = cell apoptosis
What is affinity maturation?
As immune response matures and antigen reduces, there is increased selection for B cells with higher affinity (competition)
Approximately how long does it take for antibodies to appear after antigen exposure? Which antibody is usually produced more at this point?
~1 week
IgM
Why will you have a stronger. more efficient response to a secondary exposure to the same antigen?
Memory cells
Class-switching and affinity maturation (somatic hypermutation) has already occurred
