Humoral Immunity: Antibodies And The Life Cycle Of B Cells Flashcards
What is the basic structure of a antibody/immunoglobulin?
Variable reigon - different
Constant reigon - same for all same class
Antigen binding site
Light - 2 types make two chains
Heavy - 4 chains (9 different types of chains including variants of alpha and gama chain) that make 2 chains
What are the five classes of heavy chain?
Micro
Delta
Alpha
Gamma
Epson
What are the two domains of the light chain?
Variable region and constant region
NH3 nd on variable reigon
COO- on the control end
How are the variable region of the the light and Harvey chain different?
They have different shapes
What are the four different antibody functions?
Virus and toxin neutralisation - prevents pathogen host binding
Opsonisation + ADCP - phagocytosis
Complement fixing/ MAC formation (CDC) - phagocytosis or lysis
Opsonisation + ADCC - NK induced apoptosis
What are the different classes of antibodies?
IgG - iconic antibody structure
IgD
IgE
IgA - has a J chain and a secretory component - secreted in mucus and good for respiratory infections - 2 molecules
IgM - has a J chain - 5 molecules together
What defines the different classes of antibodies?
The heavy chain
What heavy chain does igM have?
Micro/mu
What heavy chain does IgD have?
Delta
What heavy chain does IgG have?
Gamma
What heavy chain does IgA have?
Alpha
What heavy chain does IgE have?
Epsilon
What is the function of IgM?
Main Ab of primary response
Best at forming immune complexes and fixing compliment
What is the function of Ig D?
BCR
Indicated mature B cells
Only AB not secreted
What is the function of IgG?
Main Ab of secondary responses,
neutralise toxins,
opsonisation
80% of total Ab in serum
What is the function of IgA?
Secreted onto mucous, tears, saliva, colostrum
What is the function of IgE?
Allergy
Anti-parasites
Smallest percentage of Ab in serum
When can heavy chain class switching occur?
Only effects heavy chain constant region
Different effector functions
Minor: Differential splicing (mRNA level) - IgM and IgD
Major : DNA recombination
- igM to IgG, IgA, IgE
- IgG to IgA, IgE
What is class switch recombination (CSR)?
- Cytokine signal
- Switch regions
- AID and DSB repair
Recombination between switch regions
Switching only proceeds down stream
Why does antibody class switching occur?
In response to antigen stimulation and costimulatory signals
Describe a overview of the B cell life cycle
Antigen independent stage (BONE MARROW)
- Stem cells differentiate to pro B cells
- then pre B cells when recombination makes the heavy chain variable region
- immature B cell forms after light chain develops
- expression of igM and igD by differential splicing
Antigen dependent stage (BLOOD STREAM AND LYMPH NODE)
- reside in spleen until pathogen is detected
- class switching occurs
- receptors because igG or igA
- some will turn into plasma cells
- memory b cells remain in blood
What is somatic recombination ?
Alteration of the genetic information at DNA level
- VDJ recombination
- Tdt nucleotide addition
- somatic hypermutation
- class switching
What is differential splicing?
Changes made on the mRNA
- igM and igD
- membrane bound and secreted ig
What happens on the life cycle of a B cell?
- Stem cell is converted to pro-b cell (heavy chain develops)
- Pro b cell becomes a pre-b cell (light chain develops
- Immature B cell
- Quality control occurs with differential splicing of igM nd igD - mature B cells form
- Mature B cells. Then leave the be marrow and enter the spleen and lymph node
How many antibody genes are inherited?
None! No complete genes re inherited, only gene segments
The gene segments in different combinations generate many ig sequences
What re the three genetic loci encoding ig?
Two for light chain - kappa and lambda locus
One for heavy chain
Located on different chromosomes
What are. Recombination signal sequences (RSS)?
Conserved sequences upstream or downstream of gene segments
What is the on-turn/two-turn rule (12/23 rule)?
Recombination only occurs between 12bp spacer and 23bp spacer
What are the number of different combinations the antibody Haines can have/
1.98 x 10^8
What are the processes that can abuse antibody diversity?
Multiple germaline V,D and J gene segments
Junctional flexibility
P nucleotide addition
N nucleotide addition
Combination association of heavy and light chains
Somatic hypermuutation during affinity maturation
What is junctional diversity?
Junctional flexibility during VDJ recombination, P and N nucleotide recombinations
Good for antibody diversity
Bad: non-productive rearrangements - wasteful process
Describe the mechanism of junctional diversity?
- RAG1 and RAG2 bind to the terms on either end pulling them together to form a hairpin structure
- DNA is nipped and forms a hairpin at the ends of the gene set
- Enzymes process these ends and form a coding joint of the desired genes and a signal joint
- Signal joint contains other genes that were between the two gene segments
5.
Describe what occurs during hair opening and joining
The hairpin opens using the enzyme Artemis
And the DNA ends will be processed by exonuclease and TdT
The DNA ends are Joined by a series of enzymes
What is junctional flexibility?
Removal of nucleotides between gene segments during VDJ recombination
Exact mechanism unknown
Involves Exonuclease - removes mismatched nucleotides
Describe roughly what occurs during junctional flexibility
- two different hairpins are cleaved at a random parts
- Leading for one hairpin to turn into 2 DNA strands
- P Nucleotides and N nucleotides are added to the ends of the DNA strand
- The strands from each hairpin at the start join by terminal deoxynucleotidyl transferase (Tdt)
- Forming 2 long DNA strands that contain one P Nucleotide and one N nucleotide
What is allergic exclusion?
Two copies for each ig gene - one paternal an one maternal
Both genes are expressed
Antibody genes different - only on heavy chain and one light chain allele Is expressed
What is the order of rearrangement of allelic genes?
Heavy > kappa > lamba: 1st allele then 2nd allele
Describe what occurs during the Life cycle B cells
- Lymphoid progenitor cells in the bone marrow is converted into a mature B cells and T helper cells
- Mature B cells enter the spleen + lymph node and blood
- T helper cells migrate to the thymus
- Once the mature B cell enters the spleen it enters the germinal center
- In the dark zone of the germinal centre the mature B ell undergoes clonal expansion + somatic hyper mutation
- In the light zone the cells undergoes selection + class switching occurs
- After this the mature B cells now have Ig on their surface and leave the spleen/lymph node an enter the blood
- The B cells then differentiate into memory cells or plasma cells
What occurs in T cell INDEPENDENT B cell activation?
- Antigen on pathogen binds to B cell
- Triggering clonal expansion and IgM secretion
What occurs in T cell DEPENDENT B cell activation?
- Antigens on pathogen bind to dendritic cells
- Dendritic cells then bind to T helper cells
- The T helper cells then bind to B cells using (TCR + MHC2) (CD40 + CD40L)
What are the 3 signals required for B cell activation?
- Antigen binding to BCRs
- Co-stimulation by activated Th cell specific to some antigen
- Th Cell-deprived cytokines
How are B cells activated by the T cell transduction pathway?
The B cell receptor binding activates tyrosine kinase, which phosphorylated proteins
Signal transduction pathway for cell proliferation, differentiation and survival
Define affinity maturation
To fine tune antibody affinity to antigen
What occurs during clonal expansion?
- B cell that can bind to the pathogen the best will get activated
- B cell then makes clones of itself
- B cells then undergo finite maturation
What occurs during affinity maturation?
Process to. Improve affinity of antibody to antigen
Antibody binds at low affinity - antibody takes longer to bind or binds loosely
Once B cell is activated mutations occur in the antibody which allow it to have higher affinity - shapes the area of the variable region to fit the antigen better
Where does the affinity maturation take place?
In the germaline centre of the lymph node
What are follicular dendritic cells (FDC)?
Not your normal dendritic cells but are antigen presenting
Are present n the germalne centres of the lymph node
When does class switching occur?
In the light zone of the germinal center of the thymus
Occurs to the B cells with the highest affinity after affinity maturation
What is somatic mutation?
Point mutations that occur in the clonal expanded B cells
Describe how antibody finite changes through the cycles of affinity maturation
The antibody finite increases with the number of rounds of affinity maturation
