Humoral Immunity Flashcards
Describe antibodies?
Antibodies are Y-shaped molecules made by plasma cells made to fight foreign pathogens and cancerous cells
They work by blocking the pathogens from entry or tagging them for removal by other immune cells
There are two heavy and two light chains
The heavy chains have 4 domains and can be divided into m, d, g, a and e classes
The light chain can be divided into a lambda or kappa chain
The first domain of the heavy and light chain together form the variable region
- This region is the targeting system of the antibodies
- It very specific
The rest of the antibody is called the constant region which does not vary between antibodies
What are the two versions of antibodies?
There are two versions of antibodies:
- The membrane bound version, also called the B cell receptor
- The secreted form
The secreted form is the final fully functional form of the antibody, secreted by mature plasma cells
Before it gets to that stage, it is anchored on the membrane B cells for weapon development
What is the molecular structure of antibodies like?
There are disulphide bonds between the chains
CHO complexes on either side to promote interaction between antibodies and immune cells
A hinge region to provide flexibility
Complementarity Determining Regions
- These are where the antibody interacts with the antigens
The light chain and heavy chain variable regions are very different
L1, 2 and 3 and H1, 2 and 3 each separated by scaffolding
What are the 4 main functions of antibodies?
There are generally four functions that an antibody can play to combat pathogens:
The variable fragment can bind to the part where the pathogen does to the host cell and prevent the pathogen from entering the cell
It can also bind to active sites of toxins to neutralise them
Antibodies also play a role in opsonisation, the tagging of pathogens so that it becomes more visible to other immune cells such as macrophages
- The variable region will bind to the antigen while the constant region will interact with the Fc receptor of the macrophages and recruit them to perform antibody-dependent cellular phagocytosis or recruit NK cells to perform antibody-dependent cellular cytotoxicity
Antibodies can form immune complexes
- These are large clumps which are made up of the antibody and the pathogen which will be removed by other cells
- The complex can also involve complement molecules such as c1s, c1q and c1r
- When this happens the antibody is set to fix complement which will lead to a series of events promoting inflammation, phagocytosis and lysis
What are the different classes of antibodies and what are their characteristics?
IgG, IgD, IgA, IgE and IgM
Each antibody class expressed a different heavy chain constant region but variable region is the same for antibodies produced in the same b cell
IgG is the standard structure
IgD has a longer hinge region
The epsilon chain in IgE has five domains
The alpha chain of IgA and the mu chain of IgM are similar to IgG however they have tail pieces at the end of CH3 to facilitate polymerisation and joining to J chain
So secretory IgA is two IgAs joined by a J chain and a secretory component is wrapped around it; good for respiratory infections
IgM is made up of 5 monomeric IgM molecules joined together by a J chain
What is heavy chain class switching?
Only affects heavy chain CONSTANT region Enables different effector functions – deal with different pathogens Two types of class switching: Minor: Differential splicing (mRNA level) - IgM and IgD (last lecture) - Does not affect the DNA of the B cell Major: DNA recombination - IgM to IgG, IgA, IgE - IgG to IgA, IgE
What mechanism governs major class switching?
Major class switching uses a mechanism called class switch recombination (CSR)
Requires three things:
1) Cytokine signal
2) Switch regions
3) AID and DSB repair proteins
Recombination will between switch regions
Switching only proceeds downstream
- IgM to IgG, IgA, IgE because the mu gene segment is before gamma, alpha and epsilon
- IgG to IgA, IgE
Give an example of the CSR mechanism?
A B cell has received signals to switch to IgA1
So with the help of AID and other proteins, the two switch regions in front of the mu segment and the alpha segment are pulled together
Then this part will be cleaved and joined into a switch DNA circle while the heavy chain loci will also be re-joined
The VDJ region plus the first constant gene segment, in this case the alpha1 segment, will be transcribed and translated
What is secreted Ig like?
The secreted form is generated by plasma cells and the membrane form is present on the cell since the B cell is an immature B cell
The difference is the secreted version has a tailpiece while the membrane-bound version has a trans-membrane region and a cytoplasmic tail as an anchor
How does differential splicing create membrane-bound and secreted immunoglobulins?
There is a stop codon and a Poly A tail after the genes coding for the tailpiece
This is then followed by M1 and M2 coding for the trans-membrane region and the cytoplasmic tail and another stop codon and Poly A tail
This whole region will be transcribed into RNA and this whole region spliced forming mRNA to code for secreted antibodies
On the other hand to make them membrane-bound the whole region up to the second PolyA tail will be transcribed and the tailpiece segment will be spliced out
Give an overview of B cell development? (IK its really long I’m sorry)
There are two stages to B cell development:
- The antigen-independent stage
- The antigen-dependent stage
The B cell starts as a stem cell in the bone marrow
The DNA of the pro-B cell will undergo D-J and V-DJ recombination to permanently code in the heavy chain variable region
The variable region will be expressed with the mu heavy chain which is the default first heavy chain
When the cell expresses the heavy chain it is the Pre-B cell
The pre-B cell will then undergo another V-J recombination to permanently code in the light chain variable region and the constant region to become immature B cells
These cells express IgM and will mature over time
Once they can express IgM and D on their surface through differential splicing of the mRNA they will become mature B cells and circulate between the blood stream, spleen and the lymph nodes
When the B cells become activated they will undergo affinity maturation in the germinal centre, only the best will survive
By this time they will figure out what type of pathogen has invaded and undergo class switching
The majority of B cells will further develop into matured plasma cells that secrete the antibody that they coded for
While this occurs some of the B cells coding for the IgM will differentiate into plasma cells secreting IgM
After the infection some of the B cells will remain as memory B cells
How can more than 1B different B cells be produced?
During the V-DJ and V-J recombination additional diversity can be generated through junctional flexibility and P and N nucleotide addition
These mechanisms, random in nature are enough to produce more than 1 billion different B cells
What genetic loci encode Ig?
Three genetic loci encoding Ig
Two for light chain: kappa (κ) and lambda (λ) locus
One for heavy chain
Located on different chromosomes
How does VJ recombination occur to produce the light chain?
In front of each V segment there is a leader segment so that the cells know where the protein is going to end up
The V and J segments are randomly chosen to form a leader V+J along with the constant region
This is then transcribed into mRNA
Then the mRNA is translated into amino acid sequences of the light chain
Then it is folded and the leader protein is cleaved when it reaches where it needs to be forming a Kappa light chain
What are RRSs?
V(D)J recombination mechanisms involves use of recombination signal sequences (RSS) – conserved sequences
upstream or downstream of gene segments
The RSS are made up of ‘turns’ consisting heptamer and nonamer with a 12 (one turn) or 23 bp (two turns) spacer in between
