B cells and antibodies -lecture 6 Flashcards
lecture 6
how are heavy and light chains of Ig held together
by disulphide bridges and non-covalent forces.
structure of light chains
1 constant and 1 variable domain, present as 2 types: kappa and lambda. each of which undergoes VDJ rearangement
structure of heavy chains
1 variable and 3 (IgG,A and D) or four (IgMand E) constant domains.
5 main classes of AB?
IgA D E G and M. their different heavy chains determine their function.
hinge region?
the part of the AB that gives it flexibility for interaction, also present at the V-C junction.
what forms the antigen binding site
the interaction between the heavy and light chain variable domains.
variability in the AB is concentrated where?
in three complementarity determining regions = CDRs. CDR3 is more variable than CDR1 or 2.
they appear as loops at the surface of the molecule and form a variable surface for interaction with antigen.
- the binding sites can be tight pockets, grooves or extended surfaces.
what’s somatic recombination
the process during B cell development whereby multiple Ig gene segments are rearranged.
what accounts for the CDR1 and 2 loop variation
the 65 variable gene segments which code for most of the variable domain of the heavy chain.
what accounts for the variation in CDR3
the sequence variation in the diversity gene segments (27 of them) and the juncitonal gene segments (6) and in the junctions between rearranged V-D-J segments.
how do heavy chains rearrange
D joins to J the V joins to DJ. the intervening DNA is lost on recombination.
where does heavy chain rearangement occur?
on both chromosomes but if a functional heavy chain is generated, then the rearrangement of the other chromosome is blocked by a process called allelic exclusion
difference between the V D and J of heavy and light chains?
light chains have no diversity segments. instead of one chain though as with the heavy one, they have two = kappa and lamba. they first rearrange kappa then if its unseccessful they rearrange the lambda chain.
what controls chain rearrangement
recombinases that recognise conserved heptamer and nonamer sequences adjacent to the V, D and J segments.
what are the 4 processes reponsible for antibody diversity
1 - different heavy and light chain combinations
2 - selection of different V,D and J segments
3 - variable addition and loss of nucleotides at the VDJ junctions = junctional diversity
4 - somatic hypermutation = point mutations in the heavy and light chain variable regions. the mutation rate is about 1 per 1000 base paris per cell division.
how are antibodies selected for their affinity
a process called affinity maturation.
how does junctional diversity take place
the loss and addition of mucleotides at VDJ junctions.
- addition by TdT (terminal deoxynucleotide transferase) (N-nucleotide addition)
- addition due to the recombination mechanism (P-nucleotide addition)
- nucleotide deletion
what happens in the primary response of a B cell to its antigen and affinity maturation
binds it with its BCR, internalises, processes, presents it to T helper cells.
- T helper cells bind and activate to produce antibody, which will initially be of low affinity.
- only B cells with receptors of the highest affinity for he antigen will continue to capture and present it to the T helper cells and thus it is these that will be continually stimulated to proliferate and will dominate.
first antibody types produced in an immune response
IgM and IgD. they are co-expressed by alternative splicing.
what is class switching
the same as isotype switching. the process by which a B cell changes the constant region of its Ig.
how are siluble antibodies made by switching form the membrane bound BCR
the choice of the polyadenylation site is changed to one before the membrane anchor domain so that the Ig produced isnt bound to the membrane as a BCR
which Ig have subclasses and what are they
IgG has 1-4, IgA has 2, thats it
which Ig can cross the placenta
IgG1,2 and 3
which Ig cannot activate complement
IgG4, IgD and IgE.
- so IgM, IgG1,2 3 and IgA1 and 2 can.
which Ig cannot bind to macrophage FcRs
IgM, IgG2, IgD
which Ig can bind to mast cells
IgE
which Ig are found in milk
all the IgG
which Ig are found in mucus
IgM and IgA1 and 2
major difference between the types of FcRs
some are high affinity and bind monovalent AbAg, others are low affinity and only bind multivalent complexes
how does IgM work if it hasnt got a macrophage FcR
activates complement very efficiently and is recognised by the C3b receptor.
the function of C3b receptors on RBCs?
to bind AbAg complexes and deliver them to the liver and the spleen for removal by macrophages
which FcR is used for NK ADCC
FcgammaRIII
- requires complexes of AbAg to provide multiple Fc regions for recognition, then tirggers release of lytic granules.
Ig and FcR vs helminths
IgE binds worm surface, recognised by FcetaRI = high affinity, then eosinophils release granules containing proteins that are toxic to helminths
FcR for phagocytosis?
FcgammaRI = high affinity - macrophage and activate PMN
FcgammaRIIa = low affinity - macrophages and neutrophils
FcgammaRIIIb = low affinity GPI linked neutrophil only phagocytosis
FcR for B cell antibody regulation
FcgammaRIIb
what does ADCC stand for
antibody dependent cell-mediated cytotoxicity
Ig and FcR for mast cell and basophil degranulation
FcetaRI and FcgammaRIII for IgE and IgG.
- degranulation of inflammatory mediators, synthesis and release of lipid and cytokine mediators
which Ig protect the mucosal surfaces
IgA and IgM
how is IgA transported across the epithelium
a receptor for polymeric Ig recognises the J chain of IgA and transports it across the cell.
- imp in milk and in gut where it prevents attachment of bacteria and toxins
IgG across placenta
transported to give the newborn immunity to many infections its likely to encounter after birth
how do B cells shut down their antibody responses
use negative feedback. the Ab will form complexes with the Ag, B cells specific for the same antigen can bind these complexes with FcgammaRIIb and the BCR and this acts as a negative signal to terminate the B cell response
define Ig affinity
the strength of the interaction between a single antibody binding site and a single monovalent epitope on an antigen. Abs often have 2 Ag binding sites and often bind 2 epitopes on the same antigen
define Ig Avidity
a measure of the strength of the interaction between Ab and Ag due to recognition of polyvalent epitopes. IgM has low affinity but high avidity due to the pentameric complexes it forms.
