L6 - A Kelly - B cells and antibodies Flashcards
Membrane associated Ig functions as the…
Membrane associated Ig functions as the B cell receptor (BCR)
Antibodies are only produced by B cells that differentiate into,,,,
Antibodies are only produced by B cells that differentiate into plasma cells
where may antibodies be found?
They may be free in plasma, bound to cells by specific (Fc) receptors, or present in secretions such as milk, mucus and sweat
describe how the functions of antibodies (binding antigens and recruiting effector function) are structurally separate
Highly variable antigen binding or variable domains allow recognition of a vast array of different structures and give the molecule specificity.
Constant domains allow the antibody to interact with other immune system effector cells and molecules such as macrophages and complement and are said to recruit effector function.
Antibody structure image
All antibodies are formed from pairs of ____ and ______ chains held together by …
All antibodies are formed from pairs of heavy and light chains held together by covalent (disulphide bridges) and non-covalent forces.
describe light hcain variability in antibodies
Light chains have one constant and one variable immunoglobulin fold domain and are present as one of two types called k and λ
describe the strucutre of heavy chains in antibodies
Heavy chains have one variable and three (IgG, IgA and IgD) or four (IgM and IgE) constant domains.
main classes on antibody?
The five main classes of antibody IgA, IgD, IgE, IgG and IgM can be distinguished by their heavy chains denoted a, 𝛿 , ε , γ, and μ
which part of the antibody determines the function of the antibody
heavy chain
where are antibodies flexible
The hinge region gives the molecule flexibility to interact with antigen. Flexibility is also present at V-C junction and is likened to a ball and socket joint.
what forms the antigen binding site?
interaction between heavy and light chain variable domains.
Variation in these domains is concentrated in three complementarity determining regions or CDRs
Antigens can bind in tight pockets, grooves or on extended surfaces which form the antigen binding site of the molecule.
which CDRs are more variable?
CDR3 is more variable than either CDR1 or CDR2.
do humans have lots of antibodies? conformations?
yep - 1011
what is soamtic recombination?
Ig genes encoded by many gene segments which rearrange during B cell developement - somativ recombination
T or F:
Heavy chains rearrange in a precise order and intervening genomic DNA is lost.
T
Heavy chains rearrange in a precise order and intervening genomic DNA is lost.
what is this order?
1st D -> J joining
2nd V -> DJ joining
Picture of heavy chain recombination
what is allelic exclusion
Heavy chain rearrangement can occur on both chromosomes but if a functional heavy chain is generated then rearrangement on the other chromosome is prevented (allelic exclusion)
how many heavy hcains can a single B cell express?
A single B cell can only express one heavy chain.
do light chains rearrange?
Light chains have no diversity segments but also rearrange, the k chain fist and then if unproductive the l chain.
can rearrangemnt occur on both chromosomes?
Rearrangement can occur on both chromosome. If successful at the first attempt rearrangement on the other chromosome is halted. ALLELIC EXCLUSION
where does recombination take place?
Remember, RECOMBINATION takes place on DNA and uses enzymes RAG1 and RAG2 to rearrange gene segments. This is NOT splicing.
4 points about VDJ joining?
- occurs on DNA (NOT RNA)
- occurs in a precise order
- D -> J joining
- V -> DJ joining
- uses RAG recombinases that recognise conserved heptamer and nonamer sequences
- results in permanent alterations to DNA (intervening DNA is lost)
what are the 4 main processes which generate antibody diversity?
describe the process of affinity maturation?
B cells with receptors for antigen internalise, process and present it to T helper cells
The T helper cells activate the B cells to produce antibody, generally of relatively low affinity (IgM)
B cells tested on FDC bound antigen
what is isotype switching?
Each B cell can also change the constant region isotype in a process termed isotype switching or class switching.
Ig type switch
since initially all express IgM and IgD - both expressed thougfh altenrative splicing.
Isotype swithcing picutre
____ _____ generates BCR and soluble antibody
Alternative splicing generates BCR and soluble antibody
Alternative splicing generates BCR and soluble antibody - picture
picture for differnt isotypes special function
which natibody gives passive iummunity to the fetus
IgG
abundant in blood plasma and can cross the placenta
summary of antibody types and effector functions
picutre
what are Fc receptors?
recetprs for Fc region of antibodies:\
determine most of the effector functions of antibodies
do different Fc receptors have different affininties?
yep
some high affinity:
(Kd ~10-9; FcgRI) and bind monovalent
antibody/antigen complexes while others (FcgRII and III) are low affinity receptors (Kd ~10-
6) and only bind multivalent antibody/antigen complexes.
which Lc receptors have the highest affinity for IgG antibody?
FcγRI
types of receptor - picture
T or F
Fc receptors are signalling receptors that generally activate cells.
T
do different cells express different Fc receptors?
yep
what is opsonisation?
Coating of the surface of an antigen to enhance phagocytosis is termed opsonization
describe opsonisation?
Macrophages and neutrophils express Fcγ receptors which efficiently recognise the Fc region of IgG antibodies, particularly IgG1.
Receptor engagement leads to phagocyte activation
which Ig can activate complement well?
IgM
can be recognised by the C3b receptor. Binding of antigen/antibody complexes by C3b receptors present on red blood cells delivers complexes to the liver and spleen for removal by macrophages.
describe ADCC - Antibody dependent cell- mediated cytotoxicity
Natural killer cells, neutrophils, eosinophils and phagocytes express FcgRIII, a low affinity Fc receptor that only recognises antibody/antigen complexes.
Ligation of this receptor on NK cells triggers release of cytoplasmic granules containing lytic enzymes.
ADCC can not be triggered by free Ig but requires complexes of antibody and antigen to provide multiple Fc regions for recognition.
describe eosinophil mediated ADCC
Eosinophils mediate a unique type of ADCC to provide defence against helminths. IgE binds to the surface of worms.
It signals through FceRI, a high affinity receptor for IgE.
Eosinophils release granules containing proteins that are toxic to helminths. Other antibody isotypes also assist in control of infection.
describe how antibodies can lead t othe release of inflammatory mediators
- Mast cells and basophils have receptors (FceRI and FcgRIII) for IgE and IgG.
- Crosslinking of these receptors signals release of inflammatory mediators into surrounding tissues.
- occurs in seconds
- associated with the release of vasoactive amines such as histamine and lipid and cytokine mediators of inflammation such as, prostaglandins, leukotrienes and TNF-a.
- This response is also triggered during allergic reactions when allergens bind to IgE on mast cells.
antibody leadsing to mobilisation of inflammatory mediators - picture
which Ig?:
protect the mucosal surfaces from infection.
IgA and to a lesser extent IgM protect the mucosal surfaces from infection.
Delivery of antibody to mucosal surfaces (respiratory, gastrointestinal, urogenital) and secretion in milk requires transport across epithelial layers.
describe how
A receptor for polymeric Ig recognises the J chain region of IgA and transports it across the epithelial cell.
How does IgA help protect newborn infants?
IgA present in milk provides an important defence for newborn infants. It remains in the gut and prevents attachment of bacteria and toxins to gut epithelia.
do neonates ahve maternal IgG in their blood stream?
The neonatal Fc receptor transports maternal IgG across the placenta directly into the blood stream of the fetus and provides high-level protection against pathogens common in the environment at birth
(yep)
describe Antibody feedback
- negative feedback
- immune complexes form
- B cells specific for the same antigen can bind
these immune complexes with both
FcgRIIB and the BCR. - This acts as a negative signal and terminates B cell responses.
- FcgRIIB receptors also help to
control inflammatory responses by antagonising the activity of activatory Fc receptors and are expressed on many cells including macrophages, neutrophils, eosinophils and mast cells.
what is meant by antibody affinity?
Affinity of antibody binding refers to the interaction between a single antibody binding site and a single monovalent epitope on an antigen. Antibodies have at least two antigen binding sites and often bind to two epitopes on the same antigen
what is meant by antibody avidity
Avidity is a measure of the strength of interaction due to recognition of these polyvalent epitopes. IgM antibodies are often of low affinity but bind with high avidity.
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