B cells and antibodies -lecture 6

Question 1

how are heavy and light chains of Ig held together

Answer 1

by disulphide bridges and non-covalent forces.

Question 2

structure of light chains

Answer 2

1 constant and 1 variable domain, present as 2 types: kappa and lambda. each of which undergoes VDJ rearangement

Question 3

structure of heavy chains

Answer 3

1 variable and 3 (IgG,A and D) or four (IgMand E) constant domains.

Question 4

5 main classes of AB?

Answer 4

IgA D E G and M. their different heavy chains determine their function.

Question 5

hinge region?

Answer 5

the part of the AB that gives it flexibility for interaction, also present at the V-C junction.

Question 6

what forms the antigen binding site

Answer 6

the interaction between the heavy and light chain variable domains.

Question 7

variability in the AB is concentrated where?

Answer 7

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.

Question 8

what’s somatic recombination

Answer 8

the process during B cell development whereby multiple Ig gene segments are rearranged.

Question 9

what accounts for the CDR1 and 2 loop variation

Answer 9

the 65 variable gene segments which code for most of the variable domain of the heavy chain.

Question 10

what accounts for the variation in CDR3

Answer 10

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.

Question 11

how do heavy chains rearrange

Answer 11

D joins to J the V joins to DJ. the intervening DNA is lost on recombination.

Question 12

where does heavy chain rearangement occur?

Answer 12

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

Question 13

difference between the V D and J of heavy and light chains?

Answer 13

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.

Question 14

what controls chain rearrangement

Answer 14

recombinases that recognise conserved heptamer and nonamer sequences adjacent to the V, D and J segments.

Question 15

what are the 4 processes reponsible for antibody diversity

Answer 15

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.

Question 16

how are antibodies selected for their affinity

Answer 16

a process called affinity maturation.

Question 17

how does junctional diversity take place

Answer 17

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

Question 18

what happens in the primary response of a B cell to its antigen and affinity maturation

Answer 18

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.

Question 19

first antibody types produced in an immune response

Answer 19

IgM and IgD. they are co-expressed by alternative splicing.

Question 20

what is class switching

Answer 20

the same as isotype switching. the process by which a B cell changes the constant region of its Ig.

Question 21

how are siluble antibodies made by switching form the membrane bound BCR

Answer 21

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

Question 22

which Ig have subclasses and what are they

Answer 22

IgG has 1-4, IgA has 2, thats it

Question 23

which Ig can cross the placenta

Answer 23

IgG1,2 and 3

Question 24

which Ig cannot activate complement

Answer 24

IgG4, IgD and IgE.

- so IgM, IgG1,2 3 and IgA1 and 2 can.

Question 25

which Ig cannot bind to macrophage FcRs

Answer 25

IgM, IgG2, IgD

Question 26

which Ig can bind to mast cells

Answer 26

IgE

Question 27

which Ig are found in milk

Answer 27

all the IgG

Question 28

which Ig are found in mucus

Answer 28

IgM and IgA1 and 2

Question 29

major difference between the types of FcRs

Answer 29

some are high affinity and bind monovalent AbAg, others are low affinity and only bind multivalent complexes

Question 30

how does IgM work if it hasnt got a macrophage FcR

Answer 30

activates complement very efficiently and is recognised by the C3b receptor.

Question 31

the function of C3b receptors on RBCs?

Answer 31

to bind AbAg complexes and deliver them to the liver and the spleen for removal by macrophages

Question 32

which FcR is used for NK ADCC

Answer 32

FcgammaRIII

- requires complexes of AbAg to provide multiple Fc regions for recognition, then tirggers release of lytic granules.

Question 33

Ig and FcR vs helminths

Answer 33

IgE binds worm surface, recognised by FcetaRI = high affinity, then eosinophils release granules containing proteins that are toxic to helminths

Question 34

FcR for phagocytosis?

Answer 34

FcgammaRI = high affinity - macrophage and activate PMN

FcgammaRIIa = low affinity - macrophages and neutrophils

FcgammaRIIIb = low affinity GPI linked neutrophil only phagocytosis

Question 35

FcR for B cell antibody regulation

Answer 35

FcgammaRIIb

Question 36

what does ADCC stand for

Answer 36

antibody dependent cell-mediated cytotoxicity

Question 37

Ig and FcR for mast cell and basophil degranulation

Answer 37

FcetaRI and FcgammaRIII for IgE and IgG.

- degranulation of inflammatory mediators, synthesis and release of lipid and cytokine mediators

Question 38

which Ig protect the mucosal surfaces

Answer 38

IgA and IgM

Question 39

how is IgA transported across the epithelium

Answer 39

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

Question 40

IgG across placenta

Answer 40

transported to give the newborn immunity to many infections its likely to encounter after birth

Question 41

how do B cells shut down their antibody responses

Answer 41

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

Question 42

define Ig affinity

Answer 42

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

Question 43

define Ig Avidity

Answer 43

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.