Antibodies and gene rearrangement Flashcards
What is the major distinguishing difference between innate and adaptive immunity?
Innate immunity is driven by lymphocytes (e.g. B and T cells)
What makes B and T cells unique from all other cells? What does this allow them to do?
They have a genetic locus /\ It can re-arrange itself and re-combine
How does the adaptive immunity affect the immune response as it is exposed to antigen over time?
The immune response gets stronger and shorter over time for the same pathogen
How does the adaptive immune response change as it is exposed to antigen over time?
The affinity of B cells towards antigens increases
When someone is first vaccinated how is it characterised? How does the antibody respond to this?
A rise in antigen specific low affinity serum IgM /\ It takes about two weeks for IgM to respond, quickly decays away as well
When someone is given a booster vaccine ,how is it characterised? How does the antibody respond to this?
Antigen specify high affinity serum IgG /\ In about three days the antibody IgG (different one this time) increase and stays increased
What is an example of a vaccine is very effective? How ofter do people need to be immunised against it? What pathogen does it protect against?
Tetanus Toxoid (TT) /\ Single immunisation then booster every 10 years /\ Clostridium tetani (common soil bacterium FYI)
What kind of adaptive immunity are we born with?
A massive repertoire of B and T lymphocytes
What does each lymphocyte represent?
A different antigen specificity
How and when are the B and T cells produced?
Randomly produced by rearrangement of the genes coding for the Ba and T cell antigen before you are born
When did adaptive immunity develop? In what animal did it most likely develop in?
~500mya in jawless vertebrates/fish (Agnathans)
How do we know that adaptive immunity evolved from these species? What does this indicate?
It is seen in all other species beyond jawless fish showing it must have been a strong influencer of selection
What did adaptive immunity develop from?
Transposase
What is transposase?
An enzyme that cuts and shifts the transposon (a chromosomal segment that can be moved around)
How did transposase develop the adaptive immune system?
A transposon inserted into a primordial receptor gene which then caused the transposase to move out of the receptor gene enabling it to operate in trans (means acting from a different molecule FYI)
Is the transposase still active in species today? How do we know?
Yes /\ There are two transposase, Recombination Activation Genes 1 and 2 (RAG1 and RAG2) and the Recognition Sequence (RS) are still present in every species with adaptive immunity
What are RAG1 and RAG2
The ancient transposase that are still preset that cut out and shift the transposon
What is the RS? Where is it found?
Base pair sequences /\ Found at the end of all gene segments that are recognised by RAG1 and 2
How much do the RAG1, RAG2 and RS vary between species?
It doesn’t vary at all, all the same
What is the immunoglobulin fold/domain structurally?
It is made of two anti-parallel ß sheets made up of 7 constant or 9 variable ß strands that are on a 30º twist forming a ß barrel structure. A single disulphide bond between the sheets stabilises it. The ß strand are connected by loops
What is anti-parallel?
When two molecules run parallel to each other but in opposite orientations
What are the pictures below showing?
Beta strand /\ Beta plate /\ Beta barrel
What do the ß strands allow for in the Ig domain?
Allows it to be very soluble and allows the loops attached to vary their amino acid sequence as they are not constrained
How do the unconstrained loops on the ß barrel affect the structure? Why/why not?
They don’t as the loop are not constrained in the structure so changes in the amino acids of the loops doesn’t affect the structure
How big is an Ig domain?
12.5 kD (kilo daltons), ~120 amino acids
What are constant and variable ß strands?
variable domains have 9 ß strands and constant domains have 7 ß strands
What is the structure of an antibody?
Two heavy chains (50-75kD each) joined together and a light chain joined onto the end of each heavy chain (25kD)
How are the chains joined together?
The heavy chains are bonded by disulphide bonds /\ The light chains are joined to the heavy chains by disulphide bonds
Label the diagram, what is this disagree showing?
Shows Immunoglobulin G (IgG)
What does Fc stand for in the antibody structures?
Fragment crystalline
What does Fab stand for in the antibody structures?
Fragment antigen binding
How do secretory IgA and IgM exist?
IgA = dimer of IgG (e.g. 2 repeating units of it /\ IgM = pentameric of IgG (e.g. 5 repeating units of it)
Why is Fc called that?
The Fc region crystallised by itself when separated from Fab therefore fragment crystalline
What is the function of the Fc? Why this?
It is the effector /\ It is the region the antibody binds to the receptor and where the first component of the complement cascade binds to
What is the primary default antibody made by naive B cells?
IgM
What is a naive B cell?
The B cell when it is first produced
When would B cells change from producing IgM to another antibody? What would it most likely switch to? What changes in the B cell to do this?
Once there is antigen activation /\ Most likely IgG /\ The γ gene (gamma) is switched on
What is the antibody with the lowest percentage of the blood? What does this antibody do?
IgE /\ Atopic allergies
What is the structure of IgM? How many antigen binding sites does it have?
Five monomer Ig chains found together my a J chain in the middle /\ Has 10 binding sites
What is affinity? When does this occur?
The attraction per molecules that is made for other molecules /\ When the attractive forces exceed the repulsive forces
What is avidity?
The overall strength of the binding
What makes IgM effective as a primary immune response for naive B cells?
It has 10 random binding sites so it has high avidity
When the B cells first encounter a new antigen, what is its affinity? Why?
Low affinity when first encountered /\ This is because the B cell has not seen this before so the IgM has not adapted to dealing with this particular molecular pattern
How does the IgM adapt to new pathogens? How does this improve its ability to respond to antigens?
It changes into a crab like shape /\ This exposes the Fc region enabling the complement to bind to pathogen
What are the properties that make IgM good as a primary surveillance molecule for unknown pathogens?
Low affinity but high avidity –> not specialised attraction to antigens, broad range of molecular patterns it can recognise
What are the five classes of immunoglobulins? What is structurally different about each one?
IgM, IgG, IgD, IgE and IgA /\ They each have different heavy chains (e.g. different Fc region)
Where is IgA found?
On mucosal surfaces and in the breast milk
What antibodies does the mother provide to their child? How are they delivered to the infant?
IgA and IgG /\ IgA via breast milk and IgG through the placenta
Why is breast feeding important for an infants immunity?
They do not have their own immunity early on and rely heavily on the mother for their immunity, especially important as they are at high risk of infection
What immunes response is IgE involved in? What is its concentration in the serum? Why are its effects so potent?
Atopic allergies /\ 0.002mg/ml /\ Mast cells have a very high affinity for these so when IgE is activated by allergic response, they bind to mast cells and degranulate releasing their cytotoxic chemicals (e.g. histamine) causing immune response
Which antibodies activate complements?
IgG and IgM
Which antibodies are membrane bound?
IgM and IgD
What are the serum concentrations of all the antibodies?
Per mg/ml: /\ IgG = 20 /\ IgM = 6 /\ IgA = 13 /\ IgD = 0.1 /\ IgE = 0.002
What antigens can an antibody have complementarity between? Why?
Virtually any antigen /\ the amino acid diversity at the antigen binding site is vast
When does affinity arise?
When the sum of attractive forces exceeds the sum of repulsive forces
What is complementarity?
When there is a net overall attractive force between an antigen and antibody
What normally happens to the water between the antigen and antibody surface? What does this result in?
It is normally expelled /\ Strong bonds between predominantly hydrophobic amino acids
Are the net attractive forces large between particles that have a high affinity? Why? What does this mean for the particle disassociation?
Not necessarily /\ It is to do with the attractive force per molecule /\ They are very hard/never disassociate
What is the necessary solution of antibodies which have the highest affinity to reach equilibrium?
picomolsL-1 (10^-12)
What are the necessary concentration of most antibodies to reach equilibrium? What is equilibrium?
Most interaction need 10^-6 to 10^-9 /\ When 50% are bound and 50% are unbound
How many different antibodies types are in the human body? How many genes are there in the human body? How is this possible?
10^11 + /\ ~30,000 /\ Recombination in the Ig locus
How are the Ig and TcR (T - cell receptor) segmented?
Into germline gene segments
What are the germline gene segments?
Variable, Diversity, Joining and Constant
What is in the constant germline segment? How do these vary between people?
C segment contains the heavy chain gene coding (e.g. genes that code for the heavy chains of IgM, IgA etc.) /\ Very little variation
How many segments are in the V, D and J segment?
V = 100+ /\ D = 27 /\ J = 6
What is the first thing that happens when a lymphoid precursor starts to become a B cell?
Rearrangement of the V, D and J segments
How are the V, D and J segments rearranged?
RAG1 and RAG2 splice out the segments and move them
How are the segments moved between each other?
D segment joins onto end of the J segment, V segments move to end of D segment /\ FYI The H gene locus (FYI this is what its called) looks like: /\ V —— D —– J —– C
Where does most of the diversity of the antibodies come from?
The imprecision of the joining process the V, D and J segments onto the C segments
Of all the different combination of antibodies, what percent of them will most likely be useful? Why this amount?
1% /\ Most will be unstable, won’t match and interfere with other structures
How do your genes know, before birth, what type of antibody will be needed to neutralise vaccines?
They don’t, the immune system however just produces so many random antibodies that some will have affinity for the antigens
How do low affinity B cells with IgM receptors develop high affinity IgG in the face of time and persistent antigens?
Low affinity B cells are produced by stochastic (random) rearrangement so some will have a higher affinity towards antigens /\ individual antibodies are then selected by the germinal centres in the lymph nodes to be cloned and these create further variation due to the stochastic rearrangement resulting in some clones which will have higher affinity, this continues until a very high affinity B cell antibody clone is produced /\ The high affinity B cell either then turns into a plasma cell which produces high affinity IgG or into a memory cell (reside in lymph nodes and survive well past the initial infection FYI)
How do the memory cells affect the immune response?
It allows for the IgG count to increase much more rapidly than when naively challenged
