Humoral Immunity: Generation of Antibody Diversity Flashcards
Describe the structure of antibodies
2 heavy, 2 light chains
Heavy:
- 4 domains of γ,𝜀,ẟ,µ,𝜶,
- Subtypes of γ1-4 and 𝜶1-2
- Total 9 HC regions possible
Light:
- 2 domains of kappa, lambda
What is the variable region of antibodies formed of?
Variable region formed of the first domains of light and heavy chains and bind antigens specifically, CH1 supports
What is the role of the constant region?
Constant region - same for all Ab of same class
Effector functions (activating complement, binding phagocytes) CH1,2 & 3 are the constant heavy domains
What is the role of disulphide bonds in antibody structure?
Disulphide bonds stabilize VH-CH1-CH2-CH3 heavy and light chain bonds
What is the role of the antibody hinge region?
Hinge region provides flexibility and movement to structure
What is the significance of carboxylic groups on the CH2 domains?
CH2 carboxylic groups act as anchors for immune cell interactions
What is the CDR?
Complementarity Determining Regions
The CDR is where the antibody interacts with antigens on VH and VL
What is the roel of the CDR?
CDR binds to antigen (fingers and apple)
CDR3 most variable
What are the 4 main functions of antibodies to combat pathogens?
- Opsonization
- Neutralisation
- Complement / MAC
- Apoptosis
What is opsonisation?
Tagging pathogens to make them more visible to immune cells (macrophages & NK cells)
Outline how antibodies cause opsonization
- Variable regions bind to pathogen
- Constant domains bind to FcR on macrophages
=> antibody dependent cellular phagocytosis (ADCP) of smaller pathogens
Or NK cells to produce antibody dependent cellular cytotoxicity (ADCC) by releasing chemicals to induce apoptosis (infected/cancer)
How do antibodies aid neutralisation of toxins?
Variable fragments can bind competitively to viral docking sites on cells / toxin active sites; neutralise
How do antibody immune complexes fight pathogens?
Form immune complexes composed of antibodies + pathogens that agglutinate and are removed by other immune cells
How do complement proteins aid immune response?
Can involve complement molecules (C1q,s and r) that promote inflammation, phagocytosis and MAC formation (damage membrane) to cause lysis
How many antibody classes are there?
5 different classes of antibody with different functions
What is the difference between each antibody class?
Each antibody class expresses a different heavy chain constant region
But the light and heavy chain variable regions remain the same for antibodies produced from the same B cell
What is the IgG antibody structure?
IgG has the canonical structure mentioned above with 4 domains in a gamma chain
Describe the IgD structure
The delta chain in IgD has a longer hinge region
What is IgE structure like?
The epsilon chain in IgE has 5 domains
What enables IgA and IgM to polymerise?
The alpha and meu chains in IgA and IgM are similar to IgG but they have tail pieces at the end of CH3 to facilitate polymerisation and joining to J chains
Describe the structure of IgA
Secretory IgA is 2 monomeric IgA joined by a J chain; secretory component wraps around enabling it to be secreted into the mucous → good for respiratory infections
Outline the structure of IgM
IgM is composed of 5 monomeric IgM structures joined together by a J chain
How is antibody class determined?
The heavy chain variable regions and light chain are fixed by VDJ recombination
Which is the heaviest Antibody?
IgM has highest mw; pentamer and IgA is also larger; dimer
Which is the main Ig in serum?
IgG is the main antibody in serum followed by IgA
Which antibodies are able to fix complements?
Only IgM and IgG fix complements
Which antibody is able to provide immunity to foetus?
Only IgG can cross the placenta to provide immunity to the foetus
What is the role of heavy chain class switching?
Provides different effector functions to deal with different pathogens
Only affects Heavy chain constant regions
What are the 2 types of Heavy chain class switching?
Minor: differential splicing (mRNA level)
- IgM and IgD
- Doesn’t affect B cell DNA
Major: DNA recombination
- IgM to IgG, IgA , IgE
- IgG to IgA, IgE
What causes class switching to occur?
Class switching occurs due to presence of chemical signalling from cytokines released by T helper cells
What is the main signal to initiate clas switching?
CD40L on T cell interacts with CD40 on B cells and cytokine signalling
Name the mechanism for major class switching (DNA)
CSR - class switch recombination
Outline how class switch recombination occurs
1) Cytokine signal
2) switch regions
3) AID and DSB ds repair proteins
What is the rule of Class Switching Recombination?
Switching only proceeds downstream
- IgM to IgG, IgA, IgE
- IgG to IgA, IgE
How do heavy chain segments recombine?
Heavy chain gene loci undergone VDJ recombination and affinity maturation
Why can’t antibodies revert back to previous IgM class after recombination?
Once switched to IgG all the segments before will be removed ∴can’t revert back to IgM
What happens to the spliced antibody regions?
Segments cleaved out form a switch DNA circle once spliced
What are the 2 types of antibodies?
2 types of antibodies:
- membrane bound (BCR)
- Secreted form
What is the secreted form of antibody?
The secreted form is the final, fully functional form of the antibody secreted by mature plasma cells
Why are antibodies anchored to B cells before secretion?
Before its secreted it’s anchored on B cells for weapon development
How do secreted and membrane bound antibody structure differ?
Secreted IgG has a tail piece while the membrane bound antibody has a transmembrane region and cytoplasmic tail anchor
Outline the components of the constant mu region before differential splicing occurs
Constant region of Cμ composed of μ1-4 + tail piece + stop codon + polyA tail followed by M1 and M2
There is also a second PolyA tail and stop codon