MCB Lecture 46 B cells Flashcards
What are the components of the adaptive immune system?
Which lymphocytes are responsible for each?
Cellular: T lymphocytes
Humoral: B lymphocytes
What are the general characteristics of the adaptive immune response? (3)
- Slow
- Specific
- Magnitude increases with exposures
What are the receptors found on B lymphocytes called?
B Cell Receptors BCR
Antibodies
Immunoglobulins
Antigen receptors
Describe the copy number and variability of the receptors on a single B cell
There is only one type of receptor on each B cell, however there are many thousands of each receptor on each B cell
What is the function of BCR?
- They recognise and bind to antigens on pathogens that enter the body.
What are the broad groups of antigens that bind to BCRs?
Lipids
CHOs
Nucleic acid
Protein
What is the name of the region on an antigen to which the BCR binds?
Give some general facts about these areas
Epitope
Antigenic determinant
Determinant
Can be linear of discontinuous
What is the function of B lymphocytes?
- Become active when the BCR binds to an antigen (clonal selection)
- Divide and proliferate
3a. Plasma cells make and secrete antibodies to further fight the infection
3b. Memory cells wait for the next time that pathogen enters the body –> more rapid response - Classical pathway of the complement system; opsonisation
Describe the locations involved in the production of B cells
- Develop from hematopoetic stem cells in the bone marrow of the major bones in the body
- Migrate to the blood
- Move into the lymphatics, where they wait until they come across their antigen
Where do B cells spend most of their time?
50% mucosal associates lymph tissue
40-50% lymph nodes
2% blood
What are the divisions of lymphoid tissues?
List some of the organs in each
Primary lymphoid organs: where they develop Bone marrow, thymus Secondary lymphoid organs: where the lymphocytes reside when they become activated Lymph nodes Tonsils Adenoids Mucosal associated lymphoid tissue Peter cells of S.I. Large intestine
Describe what is meant by Tolerance
This is the removal of auto reactive antigens; those that respond to self antigens
How are autoimmune diseases caused?
This is when auto reactive antibodies are not removed, and an immune response is launched against part of oneself
How do lymphocytes come in contact with antigens?
The ECF in tissues drain via afferent lymphatic ducts into the lymph nodes
It is here that the lymphocytes come in contact with antigens
Describe how B cells become activated
What is the analogy?
Where does this occur?
Clonal selection
A B cell expressing a unique BCR comes in contact with an antigen that matches the receptor.
The epitope binds lick a key in the BCR lock
This occurs in the secondary lymphoid tissues
What are the features of clonal expansion? (2)
Takes time
Lymphocyte divides and proliferates
Lots of ER now present in the selected cells for protein synthesis
What two types of cells are produced in clonal expansion of B cells?
Memory cells: remain in the lymph for the next time the same pathogen invades
Plasma cells: these synthesise and secrete antibodies (the same one as the B cell first expressed)
Differentiate between the antigens that activate B and T lymphocytes
B cells: free floating antigens interact with the BCR
T cells: antigens bound to MHC interact with TCR
Where does clonal selection occur?
In the secondary lymphoid tissue
What is the function of memory cells?
These remain in the lymphatics for the next time that antigen gets inside the body
What is the function of plasma cells?
These synthesise and secrete antibodies
Describe the function of antibodies
Antibodies coat the pathogen by binding to the antigen
Removal of the pathogen
Describe briefly the structure of antibodies
Two chains: heavy and light
There are two of each type of chain per antibody
There are two regions:
Fc: constant region
Fab: Variable region
What is the Fc region?
What different forms can it take?
This is the constant region of the antibody
There are 5 different isotopes
What is the Fab region?
What different forms can it take?
This is the variable region of the BCR
This is the region that binds to the epitope of the antigen
It can take an almost infinite number of forms
What is the structure, function and location of IgA?
Dimer, J chain
Secreted over the mucosa into the gut for example
Function:
Prevents pathogens from binding to the mucosa
Binds to bacteria, toxins and viruses (via epitope)
What is the structure, function and location of IgM?
Pentamer, j chains and disulfide bonds
Present on the surface of naive B cells
Function
Neutralises toxins
Triggers the classical pathway
What is the structure, function and location of IgE?
Monomer
Bound to Mast cells
Function
Triggers degranulation: release of toxic substances from granules that kill the pathogen
What is the structure, function and location of IgG?
Monomer
Found in plasma and tissue fluids
Binds to pathogens
The Fc part then binds to the Fc-receptor on phagocytes
Triggers phagocytosis of the bacterium
Trigger classical pathway of the complement cascade
Which isotopes are involved with activating the classical pathway of the complement cascade?
IgG
IgM
Which isotype binds to the FcR on phagocytes?
IgG
Which isotype triggers release of granules?
IgE
Which isotype is normally found as a pentamer?
IgM
Which isotype is normally found as a dimer?
IgA
Describe the different possible affinities between epitope and Fab
High affinity
Low affinity
Which isotype is found on naïve B cells?
IgM
Which isotype of Fc is secreted by B cells after they are first activated?
IgM
Describe how B cells can undergo isotype switching
This is an irreversible process
The genes of the constant region are rearranged when being transcribed in gene transcription in B cells
The Fc region is now different
IgM -> IgG
What are the features f isotype switching? (3)
- Irreversible
- Requires T cell and other external signals to determine which isotype we are switching to
- Accompanied by increased affinity
What are the two ways that an individual can obtain specific immunity?
Passive: individual given antibodies to a specific antigen directly
Eg. When someone is infected with tetanus, they are transfused with the blood of someone who has made antibodies to tetanus, so that the response can be very rapid
Active: antigen presented to BCR in the body
How do natural killer cells interact with the adaptive immune system?
NK Cells have FcR
Binding of antibodies (constant region) triggers degranulation
Which isotype is normally transferred in passive immunity?
IgG
