Introduction to antibodies and B lymphocytes Flashcards
What are antibodies produced by?
B lymphocytes
What is a surface bound antibody called?
B cell receptor
Describe the structure of an antibody
4 polypeptide chains
- 2 identical heavy chains
- 2 identical light chains
Held together by S-S bonds
Glycosylated
What do the heavy and light chains consist of?
V and C domains
What does each V and C domain do?
Folds into a characteristic 3 dimensional shape = immunoglobulin (Ig) domain
What does an Ig domain consist of?
2 layers of a beta pleated sheet held together by disulfide bridge
What is the F(ab’)2 region?
Constitutes the 2 antigen binding regions
What is the antigen recognition site composed of?
variable region of both heavy and light chain
what does the antigen recognition site determine?
specificity, affinity and avidity of interaction with the antigen
What is the Fc region?
The tail region (the part of the antibody that is not the Fab regions)
Why is the Fc region crystallisable?
It is identical in all antibody molecules of a particular type so crystallises in solution
What does the Fc region confirm?
Functional properties of antibody
What is the Fc region recognised by?
FcR
What does the Fc region bind?
Complement
What are the 4 functions of antibodies?
Complement
Opsonisation/phagocytosis
recognition killing
sensitisation/cell activation
How many classes of antibodies are there?
5 classes and 9 subclasses in humans
What are antibody classes defined by?
heavy chain constant region
Describe the structure of IgM
Pentameric stabilised by J chain
4 constant regions on heavy chain
Heavy chain encoded by mu gene
What does the monomer of IgM do?
forms BCR on most B cells in association with Ig-alpha and Ig-beta chains (acts as membrane bound receptor)
What are the properties of IgM?
Low affinity
High avidity
Activates complement (classical)
Major antibody in primary response
Describe the structure of IgG
Monomeric
3 constant regions on heavy chain
heavy chain gamma gene
What are the properties of IgG?
major antibody in secondary response
can cross placenta and protect foetus
activates complement classical and opsonin (FcR)
What are the 4 sub-classes of IgG?
G1, G2, G3, G4
Describe the structure of IgA
Heavy chain encoded by alpha gene
3 constant regions on heavy chain
What are the subclasses of IgA?
IgA1 and IgA2
Where is IgA found?
In tears, milk, saliva, sweat etc.
What are properties of IgA?
Can get localised mucosal response, different to systemic response
Protects external surfaces (mucosal immunity)
does not activate complement by classical pathway
How is IgA secreted?
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)
Why does IgA need to be transported to the lumen?
To neutralise microbial pathogens in the lumen before they invade
Describe the structure of IgD
Heavy chain encoded by delta gene
Monomeric
3 constant regions on heavy chain
Where is IgD expressed?
On the surface of B cells with monomeric IgM
What are properties of IgD?
Has specific antigen binding activity but NO effector functions
Sensitive to proteolytic degradation and heat
Involved in antigen triggered B-cell differentiation
Describe the structure of IgE
Heavy chain encoded by epsilon gene
4 constant regions on heavy chain (looks like monomer IgM)
Where is IgE found?
Majority bound to mast cells and basophils through high affinity Fc-epsilon-R1
What are properties of IgE?
Key in allergic response
Important role in parasitic infections as mast cells are very effective at killing multicellular organisms
What do most B cells express?
monomeric IgM and IgD
What do both IgM and IgD have on B cells?
Same specificity on each individual cell
What do surface bound Ig have?
Short intracytoplasmic tail and is associated with accessory molecules to form the BCR
What are ITAMs?
Stands for immunoreceptor tyrosine-based activation motifs
They are amino acid motifs found in the cytoplasmic tails
Essential for signalling
What does ligation of the BCR lead to?
Phosphorylation of ITAM molecules
What does phosphorylation of ITAM lead to?
Downstream cascade of events resulting in differentiation into plasma cell and antibody production
What is the outcome of BCR ligation?
Clonal expansion
What is clonal expansion?
When 1 B cell is activated and becomes a clone of cells, each with the same BCR specificity
What do the majority of B cells become?
Plasma cells
Describe plasma cells
Limited life-span
Apoptose after few days
What do a small proportion of the B cells remain as?
Memory cells
What is the first Ab response?
IgM
What happens in the secondary response?
Class switching to other classes of Ab, especially IgG
How does class switching occur?
Mutation of variable region sequences - somatic hypermutation
What happens after class switching?
Affinity maturation (requires T cell help)
How are monoconal antibodies produced?
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
What are the problems with monoclonal antibodies?
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
What were the solutions to the problems with monoclonal Ab?
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
What is Ipilimumab?
Anti-CTLA-4 (CTLA-4 turns off T cells)
What is Nivolumab?
Anti-PD1 (PD1 kills T cells)
Why are molecules like CTLA-4 and PD1 needed?
To prevent autoimmune responses/diseases
