Introduction to antibodies and B lymphocytes

Question 1

What are antibodies produced by?

Answer 1

B lymphocytes

Question 2

What is a surface bound antibody called?

Answer 2

B cell receptor

Question 3

Describe the structure of an antibody

Answer 3

4 polypeptide chains
- 2 identical heavy chains
- 2 identical light chains
Held together by S-S bonds
Glycosylated

Question 4

What do the heavy and light chains consist of?

Answer 4

V and C domains

Question 5

What does each V and C domain do?

Answer 5

Folds into a characteristic 3 dimensional shape = immunoglobulin (Ig) domain

Question 6

What does an Ig domain consist of?

Answer 6

2 layers of a beta pleated sheet held together by disulfide bridge

Question 7

What is the F(ab’)2 region?

Answer 7

Constitutes the 2 antigen binding regions

Question 8

What is the antigen recognition site composed of?

Answer 8

variable region of both heavy and light chain

Question 9

what does the antigen recognition site determine?

Answer 9

specificity, affinity and avidity of interaction with the antigen

Question 10

What is the Fc region?

Answer 10

The tail region (the part of the antibody that is not the Fab regions)

Question 11

Why is the Fc region crystallisable?

Answer 11

It is identical in all antibody molecules of a particular type so crystallises in solution

Question 12

What does the Fc region confirm?

Answer 12

Functional properties of antibody

Question 13

What is the Fc region recognised by?

Answer 13

FcR

Question 14

What does the Fc region bind?

Answer 14

Complement

Question 15

What are the 4 functions of antibodies?

Answer 15

Complement
Opsonisation/phagocytosis
recognition killing
sensitisation/cell activation

Question 16

How many classes of antibodies are there?

Answer 16

5 classes and 9 subclasses in humans

Question 17

What are antibody classes defined by?

Answer 17

heavy chain constant region

Question 18

Describe the structure of IgM

Answer 18

Pentameric stabilised by J chain
4 constant regions on heavy chain
Heavy chain encoded by mu gene

Question 19

What does the monomer of IgM do?

Answer 19

forms BCR on most B cells in association with Ig-alpha and Ig-beta chains (acts as membrane bound receptor)

Question 20

What are the properties of IgM?

Answer 20

Low affinity
High avidity
Activates complement (classical)
Major antibody in primary response

Question 21

Describe the structure of IgG

Answer 21

Monomeric
3 constant regions on heavy chain
heavy chain gamma gene

Question 22

What are the properties of IgG?

Answer 22

major antibody in secondary response
can cross placenta and protect foetus
activates complement classical and opsonin (FcR)

Question 23

What are the 4 sub-classes of IgG?

Answer 23

G1, G2, G3, G4

Question 24

Describe the structure of IgA

Answer 24

Heavy chain encoded by alpha gene
3 constant regions on heavy chain

Question 25

What are the subclasses of IgA?

Answer 25

IgA1 and IgA2

Question 26

Where is IgA found?

Answer 26

In tears, milk, saliva, sweat etc.

Question 27

What are properties of IgA?

Answer 27

Can get localised mucosal response, different to systemic response Protects external surfaces (mucosal immunity) does not activate complement by classical pathway

Question 28

How is IgA secreted?

Answer 28

IgA is produced by plasma cells in dimeric form in the lamina propria IgA dimers bind Poly-Ig receptors on basolateral surface of epithelial cells Complex is endocytosed Vesicles fuse with the luminal surface and receptor is cleaved by a protease Dimer is released into the lumen with part of the receptor bound to it (secretory component)

Question 29

Why does IgA need to be transported to the lumen?

Answer 29

To neutralise microbial pathogens in the lumen before they invade

Question 30

Describe the structure of IgD

Answer 30

Heavy chain encoded by delta gene Monomeric 3 constant regions on heavy chain

Question 31

Where is IgD expressed?

Answer 31

On the surface of B cells with monomeric IgM

Question 32

What are properties of IgD?

Answer 32

Has specific antigen binding activity but NO effector functions Sensitive to proteolytic degradation and heat Involved in antigen triggered B-cell differentiation

Question 33

Describe the structure of IgE

Answer 33

Heavy chain encoded by epsilon gene 4 constant regions on heavy chain (looks like monomer IgM)

Question 34

Where is IgE found?

Answer 34

Majority bound to mast cells and basophils through high affinity Fc-epsilon-R1

Question 35

What are properties of IgE?

Answer 35

Key in allergic response Important role in parasitic infections as mast cells are very effective at killing multicellular organisms

Question 36

What do most B cells express?

Answer 36

monomeric IgM and IgD

Question 37

What do both IgM and IgD have on B cells?

Answer 37

Same specificity on each individual cell

Question 38

What do surface bound Ig have?

Answer 38

Short intracytoplasmic tail and is associated with accessory molecules to form the BCR

Question 39

What are ITAMs?

Answer 39

Stands for immunoreceptor tyrosine-based activation motifs They are amino acid motifs found in the cytoplasmic tails Essential for signalling

Question 40

What does ligation of the BCR lead to?

Answer 40

Phosphorylation of ITAM molecules

Question 41

What does phosphorylation of ITAM lead to?

Answer 41

Downstream cascade of events resulting in differentiation into plasma cell and antibody production

Question 42

What is the outcome of BCR ligation?

Answer 42

Clonal expansion

Question 43

What is clonal expansion?

Answer 43

When 1 B cell is activated and becomes a clone of cells, each with the same BCR specificity

Question 44

What do the majority of B cells become?

Answer 44

Plasma cells

Question 45

Describe plasma cells

Answer 45

Limited life-span Apoptose after few days

Question 46

What do a small proportion of the B cells remain as?

Answer 46

Memory cells

Question 47

What is the first Ab response?

Answer 47

IgM

Question 48

What happens in the secondary response?

Answer 48

Class switching to other classes of Ab, especially IgG

Question 49

How does class switching occur?

Answer 49

Mutation of variable region sequences - somatic hypermutation

Question 50

What happens after class switching?

Answer 50

Affinity maturation (requires T cell help)

Question 51

How are monoconal antibodies produced?

Answer 51

Inject a mouse with antigen Extract the B cells from the mouse Fusion of these B cells with myeloma cells Results in hybridoma cells Isolate antibody from these cells = monoclonal antibodies

Question 52

What are the problems with monoclonal antibodies?

Answer 52

Most early McAb were murine - humans will recognise mouse Ab as foreign Constant region heterogeneity (not the same in mice and humans) Can produce anti-antibodies Inflammation can occur

Question 53

What were the solutions to the problems with monoclonal Ab?

Answer 53

Chimeric antibodies - retain the antigen-binding V region of murine Ab and replace the rest with human Ab Humanised antibodies - murine hypervariable regions grafted to human antibodies Fully humanised antibodies - replace Ig genes of mice with human Ab genes and immunise the mice with antigen to produce human mAbs

Question 54

What is Ipilimumab?

Answer 54

Anti-CTLA-4 (CTLA-4 turns off T cells)

Question 55

What is Nivolumab?

Answer 55

Anti-PD1 (PD1 kills T cells)

Question 56

Why are molecules like CTLA-4 and PD1 needed?

Answer 56

To prevent autoimmune responses/diseases