Adaptive immunity B cells Flashcards
LO
- Describe the functions and properties of B cells in the immune system
- Describe the processes involved in B cell development
specificity and diversity
- Describe the processes involved in B cell activation and differentiation
memory,
class switching
Compare innate and adaptive immunity
Activity of the immune system when an infection occurs
How does the immune system work?
- You have to identify the pathogen
The immune system uses many receptors to do this
- A decision needs to be made to attack
The attack is planned from tissues like the spleen and the lymph node
- You need a coordinated approach
Cells need to communicate with each other
- You need an army or soldiers to launch the attack
B cells provide humoral immunity, what is meant by this?
Humoral immunity or humoural immunity is the aspect of immunity that is mediated by macromolecules found in extracellular fluids such as secreted antibodies, complement proteins, and certain antimicrobial peptides
Where do B cells arise from?
Arise in the bone marrow of adult mammals
What do B cells recognise?
What part of the B cell recognises this?
B cells recognize native, (extracellular) antigen via B Cell Receptor (BCR)
Whats the difference between a BCR and an antibody?
B cell receptor refers to an immunoglobulin molecule which serves as a type of transmembrane protein on the surface of B cells while an antibody refers to a blood protein that the B cells produce in response to and counteracting a specific antigen
What do B cells produce?
Antibodies
Do B cells provide long or short term memory?
long term memory
Tell me a B cells response when a pathogen is present?
- to elicit a response to antigens, B and T cells must come close together
- B cell must recieve two signals to become activated
- one from the native antigen and another from the T cells cytokines
Whats a naive B cell?
What can it differentiate into and what do these secrete?
A B cell that has not been exposed to an antigen
Once exposed it becomes either a memory B cell or a plasma cell that secretes antibodies specific to the antigen that was originally bound
Memory cells do not secrete antibodies until activated by their specific antigen
Do plasma cells have BCR? Give a reason for this?
NO
They produce so many antibodies that they require all available energy for that
What do B cells need to recognise antigens?
BCR
- these are highly specific to an antigen
Tell me about BCR and antibody structure
- The BCR is a membrane bound antibody
Antibody structure:
- two light chains (25KDa)
- two heavy chains (50KDa).
- Variable (V) region
- constant region (C).
- The antigen binding sites are found within the variable regions
- The effector function activity is found within the constant region
What is the isotype of the antibody determined by?
The carboxyl terminus of the heavy chain
What are the Fab and Fc regions on the antibody?
- Fab – Fragment Antigen Binding
- Fc – Fragment Crystallizable
Label the different V and C regions of the antibody
Antibodies are hugely diverse and there are 10 billion different antibody molecules in each of use. How is this diversity generated?
Through VDJ recombination
What does VDJ recombination only occur in?
B cells at the three loci that make up the antibody molecule
What are the three loci that make up the antibody molecule
Tell me how antibody diversity is generated through the three antibody loci
- There are three different genetic loci that make up the light chains and heavy chains
- The genes that make up antibody molecules come in different segments
- These segments are not next to each other in the genome
- Segments are joined in different combinations to generate antibody diversity
- This occurs through VDJ recombination
- Most variation comes from V J and C segments
The sources of antibody diversity include the presence of multiple V gene segments, combinatorial diversity resulting from random recombination of V, D, and J segments, diversity due to insertion of nucleotides which result in amino acid changes in the V-D and D-J junctions, and the coexpression of different heavy and light chain pairs.
What does the VDJ in VDJ recombination stand for?
Variable (V) genes
Diversity (D) genes
Joining (J) genes
Tell me about the main stages of VDJ recombination?
Where does recombination occur and what are these areas made up of?
Occurs at Recombination signal sequences (RSSs)
- RSSs are made up of a heptamer – 12/23 (can have either 12 or 23 bp spacer) – nonamer (9 bp at the end)
VDJ recombination has to be able to fix what?
transcriptional orientation
Tell me about recombinations role in transcriptional orientation
Recombination joins the segments in the correct transcriptional orientation
What are the two ways that VDJ recombination can generate diversity?
1. Combinational diversity:
From the different combinations of gene segments
2. Junctional diversity:
From the addition of nucleotides when recombination occurs
Whats junctional diversity?
The DNA sequence variations introduced by the improper joining of gene segments during VDJ recombination
Tell me about the generation of junctional diversity through recombination
Where does the junctional diversity occur between antibodies?
Where do these fall?
- Antibodies have Complementarity Determining Regions (CDRs)
- CDR1 and CDR2 fall where the V gene segments are located
- CDR3 falls at the junction of V/D and J segments
- This is where junctional diversity occurs – the addition of nucleotides at the junctions
- It is the most variable loop
The diversity of antibodies
Tell me about B cell development and maturation
Where does VDJ recombination and B cell development occurs in?
The bone marrow
Tell me about B lymphocyte development in the periphery
- B cells produce IgM and IgD as soon as it matures
- Only IgM and IgD when first come into contact with the antigen
- IgM secreting plasma cells produce IgM antibodies (IgM only form multimers as opposed to monomers)
- The B cells can then class switch to IgG/A/E that secrete the isotypes of the antibodies
Tell me about B cell activation in the periphery?
- A B cell can only be activated by helper T cells that respond to the same antigen
What are the two methods in which B cells can be activated in the periphery?
- B cells respond to antigen in a thymus dependent or thymus independent manner
- Thymus dependent – The B cell requires T helper cells
- Thymus independent – No T helper cells required (still need a second signal-look up what this could be). B cell activation could occur via this method but don’t need to know much about this
Tell me the steps to thymus dependent B cell activation
Why is the CDR3 loop the most variable?
a. it includes both V and J segments
b. It includes RSS sequences
c. it includes V, D and J segments
d. it includes many V segments
c. includes V, D and J segments
The complementarity determining regions of an antibody are found in the…
a. The Fc region
b. The hinge region
c. The Fab region
d. The constant region
c. The Fab region
What does RAG do?
a. to open the DNA during VDJ recombination
b. To join signal sequences
c. To bind DNA
d. To join the constant region to the variable region
a. to open the DNA during VDJ recombination
What do Th cells express and what does this go on to stimulate?
Th cells express CD40L – stimulates B cells to proliferates and become plasma cells
What do the following do…
IL4
IL5 and IL6
- IL4 drives proliferation
- IL5 and IL6 drive plasma cells
Where does B cell proliferation occur?
In the germinal centres of the lymph nodes
What do B cells enter the lymph nodes via?
Lymphatic vessel
Whats more organised the secondary or primary lymphoid follicles?
Secondary lymphoid follicles are more highly organised than the primary
What do germinal centers have a high concentration of?
Germinal centres have high concentration of immune cells are highly organised.
What are the 4 differentiated zones in the germinal center?
- T cell zone
- mantle zone
- dark zone (DZ)
- light zone (LZ)
Tell me what happens in each zone of the germinal center?
- B cells become temporarily trapped in the T cell zone to come into contact with antigen and T helper cells (where co-stimulation with T cells can occur)
- The mantle zone is made up of resting B cells displaced by activated B cells
- Dark zone is where high proliferating B cells are
- The light zone has less highly proliferating B cells
What is the germinal center made up of and when does it form?
- The germinal center is made up of proliferating B cells
- Germinal centers form 7-10 days after an infection
Label this lymph node
In the germinal center, what do B cells undergo?
- Somatic hypermutation
- Affinity maturation
- Class switching
Tell me about somatic hypermutation?
- Further diversifies antibodies to generate a more specific antigen response
- Introduces point mutations in the V region of the light and heavy chain
- AID is a cytidine deaminase that introduces nicks in the DNA that are ‘repaired’
- Requires a single strand of DNA
Targets DNA that is being transcribed ei. Ig genes
Tell me about affinity maturation
- As antibody V regions are mutated the B cells are selected based on their ability to bind antigen
- As somatic hypermutation progresses the antibodies become more specific for antigen
Somatic hypermutation and affinity maturation
Tell me about antibody class switching
- Early in an immune response B cells produce IgM
- Class switching occurs through DNA recombination
- Changes to IgG, IgA or IgE during the immune response
- Allows a different C region to be used in an antibody that has a specific antigen binding region
What are the two locations in which generation of specificity and diversity occurs and what things occur at these locations?
Bone marrow: Antigen independent
Periphery: Antigen dependent
Tell me about the antigen independent specificity and diversity that occurs in the bone marrow?
Bone marrow
Antigen independent:
- Paring of different heavy and light chain
- Recombination of V, D and J segments
- Variability on the joins of the recombined gene segments
- P- and N region nucleotide addition
Tell me about the antigen dependent specificity and diversity that occurs in the periphery?
Periphery
Antigen dependent:
- Somatic hypermutation
- Class switching and affinity maturation
The course of an infection
State 3 antibody functions
- neutralisation
- opsonisation
- complement activation
Tell me about neutralisation
- Ab bind to toxins released by bacteria
- Ab can block viral binding proteins
- Block the adherence of bacteria to host cells
Tell me about opsonisation
Phagocytosis by macrophages and neutrophils
Will clonal expansion start before you get affinity for maturation?
yes
does affinity maturation still occur in the tertiary response?
yes
The stages from B-cell activation to antibody functions
What do Fc receptors bind to and where are they found?
- Fc receptors bind to the Fc of Ig molecules
- Found on many immune cells
NK cells, mast cells, neutrophils, eosinophils
What are Fc receptors specific for?
different antibody subclasses and isotypes
What do Fc receptors allow immune cells to target?
Pathogens
What can phagocytic cells do through Fc receptor binding?
Phagocytic cells can internalize antigen through Fc receptor binding
How will nautral killer cells (NKC) target cells?
through antibody dependent cell mediated cytotoxicity (ADCC)
What is the first antibody subclass expressed?
IgM
- Expressed before somatic hypermutation - low affinity
- Form a multimer with 10 ag binding sites
What do the following subclasses do?
IgG
IgA
IgE
- IgG opsonizes pathogens for phagocytosis
- IgA functions on epithelial surfaces - neutralization
- IgE localizes with Mast cells on the mucosa
Antibody subclasses
Surviving B cells within the germinal center differentiate into what?
Wha is this controlled by?
plasma or memory cells
Controlled by BLIMP1 – switches off proliferation and affinity maturation
Tell me about what memory B cells have that make them highly specific?
- Memory B cells have undergone affinity maturation and are long lived
- They have a BCR that is highly specific for an antigen but do not secrete antibody
Vaccination- induction of a memory response
Summary I and II
Summary
- B cells express a B cell receptor that has the same structure as a secreted antibody
- An antibody is made of 2 heavy chains and 2 light chains that form a Variable region and Constant region
- Antibody diversity is formed through VDJ recombination
- B cells develop and undergo VDJ recombination in the bone marrow
- Activation of B cells occurs in the periphery, particularly in lymphoid tissues
Summary II
- B cells present antigen to T helper cells and receive positive cytokine signals
- In the germinal centers B cells undergo:
- Somatic hypermutation
- Affinity maturation
- Class switching
- Antibodies function by:
- Neutralization
- Opsonization
- Complement
- The memory immune response is due to higher affinity antibodies
- The classes of antibodies have different roles
