Humoral Immunity; Antibodies and the life cycle of B cells Flashcards
What are antibodies made by?
Antibodies are made by plasma cells to fight against pathogens and cancerous cells. They work by blocking pathogens or tagging them for removal of other cells.
Antibodies are also called IMMUNOGLOBULINS.
What is the overall structure of antibodies?
Y shaped molecules made up of 2 heavy chains and 2 light chains.
The heavy chains have 4 domains. (with extra subtypes) The light chains have 2 domains.
All chains are a stream of amino acids.
Different antibodies have different variable regions for each pathogen. All antibodies of the same class have the same constant region. 2 types of antibody: B cell receptor and secreted antibody
What are the different classes of antibodies?
IgD- heavy chain delta, only antibody that is not secreted, B cell receptor, its presence indicates mature B cells
IgG- heavy chain gamma, main antibody in serum- 80%, only antibody that crosses the placenta, constant region of IgG usually binds to phagocytes, mainly used to fight off pathogens, main antibody of secondary responses, involved with neutralising toxins and opsonisation
IgA- heavy chain alpha, secreted into ,mucous, tears, saliva, colostrum
IgM-heavy chain mu, mainly involved in the primary response, best at forming immune complexes and fixing complement
IgE-heavy chain epsilon, constant region binds to mast cells and basophils, involved in allergies and getting rid of parasites
What does heavy chain class switching affect? What are the 2 types of class switching?
Heavy chain class switching affects only the constant region of the heavy chain
Class switching allows body to be more versatile with the pathogens it deals with
class switching: 1.minor class switching: differential splicing- between IgM and IgD (minor because it does not affect the DNA of the B cell itself)
2.major- DNA recombination
How does the B cell know which class to switch?
by sensing chemicals around them, in which these chemicals are produced by T helper cells and indicates what type of pathogen you are dealing with.
In addition to cytokine signals, also need CD40L on T cells which interacts with CD40 on B cells
What 3 things are needed for class switch recombination?
1) Cytokine signal; 2) Switch regions; 3) AID and DSB repair proteins
How do the two types of antibodies- the secreted form and membrane form- differ/ how are they the same?
The secreted form is generated by plasma cells and the membrane bound B cell receptor is present on the cell since it is an immature B cell.
Both the secreted and membrane bound antibody have the same heavy and light chain variable regions and constant regions.
The difference is the secreted version has a tail piece whereas the membrane bound version has a transmembrane region and a cytoplasmic tail as an anchor.
Summary so far
Antibodies – secreted by B cells to neutralize pathogens; B-cell receptor
Structure – 2 HC, 2LC
– domain vs fragment
– membrane-bound vs secreted
VH and VL CDRs bind to antigen
5 classes of antibodies – different effector functions to deal with different
pathogens
– Class switching: Heavy chain constant region change, the rest stays
the same
What are the 2 stages to B cell development?
The antigen independent stage and antigen dependent stage
What does the life cycle of the B cell include?
The B cell starts its life in the bone marrow. It begins as a stem cell and differentiates into a pro-B cell.
The DNA of the pro-B cell undergoes V–>J and V—>DJ recombination to permanently code in the heavy chain variable region. The variable region will be expressed with the new heavy chain.
This is now known as the Pre-B cell - the cell becomes a pre-B cell when it can express a fully heavy chain with a unique variable region- which then undergoes another V—> J recombination to permanently code in the light chain variable and constant region to become immature B cells.
These immature cells express IgM and mature over time- once they can express IgM and IgD on their surface through differential splicing of their mRNA, they will become mature B cells and circulate between the bloodstream, spleen and lymph nodes.
The majority of cells further develop into professional plasma cells that secrete the antibody they code for.
After infection, some B cells remain as memory B cells.
How can additional diversity be generated during recombination of the heavy and light chains?
During the VD–>J and V–> recombination of the heavy and light chains, additional diversity can be generated:
- through junctional flexibility
- through P and N nucleotide addition
These help form the billions of types of B cells.
What is IgD like?
The IgD is like a quality control check
How many antibody genes are inherited?
None! No complete genes are inherited, only gene SEGMENTS.
These gene segments are arranged in different combinations to generate many antibody( Ig) sequences.
What is the heavy and light chain gene loci made up of?
different segments; V, J and C segments for the light chain and V, D , J and C (simplified) segments for the heavy chain.
There is rearrangement of these segments.
What are the 3 genetic loci encoding Ig?
Two for light chain: kappa (κ) and lambda (λ) locus
One for heavy chain
Located on different chromosomes
What are recombination signal sequences (RSS)?
Recombination signal sequences (RSS) – conserved sequences
upstream or downstream of gene segments
made up of ‘Turns’ consisting heptamer and nonamer with a 12 or 23 bp spacer
there are 2 types of turns:
a two turn contains a 23bp spacer
a one turn contains a 12bp spacer
Why do you need two types of turns in regards to recombination?
One-turn/two-turn rule (12/23 rule)
Recombination only occurs between a segment with a 12bp spacer and a 23bp spacer.
ie recombination cannot occur between a one turn and one turn or two turn and two turn ie it needs to be between a one and two turn
What are some examples of mechanisms for generation of antibody diversity?
Multiple germline V, D and J gene segments
Combination V-J and V-D-J joining
Junctional flexibility
P-nucleotide addition
N-nucleotide addition
Combinatorial association of heavy and light chains
Somatic hypermutation during affinity maturation
When can junctional diversity take place? what is a good thing/bad thing?
Junctional flexibility during V(D)J recombination, P and N nucleotide
additions
Good: Antibody diversity
Bad: Non-productive rearrangements (incorrect reading frame) –wasteful process
minor hairpin formed is between 2 strands of DNA
major hairpin is whole DNA folded in half
enzymes then come to repair and process the ends of the hairpin, forming a coding joint of v17 and j3 as well as a signal joint
process from hairpin formation:
hairpin needs to be opened which is done by artemis
exonucleases/TdT then mess around with the ends of the hairpins adding/removing nucleotides
then ends are joined together again by another series of enzymes
What does junctional flexibility involve?
▪ Removal of nucleotides between gene segments during V(D)J recombination
▪ Precise mechanisms unknown
▪ Involves Exonuclease - removes mismatched nucleotides
What is allelic exclusion?
Two copies of each Ig gene – one from mother and one from father
In other cells, both genes are expressed
Antibody genes different – Only one heavy chain allele and one light
chain allele is expressed
Order of rearrangement: Heavy>kappa>lambda; 1st allele then 2nd
Ensure each B cell makes one type of antibody
Summary of antigen independent phase in the life cycle of a B cell
Life cycle of B cell – antigen-independent phase (occurs in bone marrow)
B cell receptor diversity at this stage generated by:
– Multiple germline genes from mum and dad
– Heavy and light chain combinations
– V(D)J recombination (one-turn/two-turn rule)
– Additional diversity: P and N nucleotide addition;
Junctional flexibility
Mature, naïve B cells expresses IgM and IgD (once an immature B cell expresses both IgM and IgD, it is a mature B cell)
– Somatic recombination vs differential splicing
Allelic exclusion
When infection of a pathogen occurs, where do B cells go and what process occurs there?
What does that process involve?
T helper cells are involved in the activation of B cells when a pathogen arises/enters. The activated B cells migrate to the germinal centre, GC where it will undergo affinity maturation. This is to improve affinity for attacking pathogens.
Affinity maturation involves:
Clonal expansion
Somatic hypermutation
Selection
This process is repeated several times. Then the antibody will receive signals to identify the pathogens that entered. These antibodies will then undergo class switching to ensure they have the appropriate effective functions.
After this the B cells differentiate to plasma cells, secreting antibodies whilst still maintaining some B cell receptors on their surface.
A few B cells will become memory B cells. Both B cells and plasma cells will circulate around in the bloodstream
In summary, what happens when they body encounters a pathogen?
A subset of B cells that recognise the pathogen will be activated. The rest of the B cells that are not activated continue to patrol the bloodstream.
