Antibodies Flashcards
(21 cards)
What are antibodies used in
They are produced by B cells (a differentiated B lymphocyte)
Adaptive “humoral” immunity (specific)
What features do antibodies have that are integral for participation in the adaptive response
1-Specificity
B cells are programmed to clonally produce Ig with specificity for a particular antigen or an epitope of that antigen (Ag)
Attributed to a defined region (FAB) of the Ig
Restircts the Ig to its complementary eptiopes
Large variation = large Ig diversity that can react with many different eptiopes
2-Biological activity
Attributed to the class of the antibody as well as the Ag or eptiope specificty defined by the (FC) region
Complement fixation and activation
Passive immunity (crossing the placenta)
Activation of mast cells / basophils (hypersensitivity)
Toxin neutralisation (agglutination)
What is the basic antibody structure
Within these chains there are constant and variable regions
What chains make up Igs
Light chains are always identical to each other and so are heavy chains
There is a hinge region which provides the structure with flexibility therefore letting Ag binding which could be further apart
Disulphide bonds within the chain make it a globular structure
There is not much secondary or tertiary structure
How many domains do heavy chains have
Heavy chains have 1 variable and 3 constant domains
How many domains do light chains have
Light chains have 1 variable and 1 constant domain
Specific antibody observations
Higher vertebrates produce Ig and show homology
If one species is immunised with another species Ig they will produce Igs for that other species Ig
What are key antibody characteristics
wo major light chains classes (see slide 12 for symbols)
Ratio of these in humans is 60% K
Light chains are either of these but never 1 of each simultaneously
There are 5 heavy chains (isotypes)
They differ in peptide length and CHO ( carbohydrate) content
Different biological function between types – this determines Ig isotype
Both heavy chains isotype are identical in any given Ig
What are the antibody domains
Intrachain interactions and s-s bonds hold the H and L chains together
Interchain s-s bonds and interactions form loops and “globulin” structure
Ig’s (except E and M) have a short amino acid sequence between CH1 andd CH2 on the H-region (hinge)
The hinge*
Mainly cys and pro residues
Flexibility between the 2 FAB regions – can open and close and can bind to 2 epitopes
Open so can be cleaved by proteases
*
Fc (constant) regions provide biological activity wile FAB (variable) regions provide epitope specificity
The greatest variability is the N-terminal 110 amino acids of both the light and heavy
What are complementary determining regions
There are 3 CDRs for each light and heavy chain CDR1, CDR2 and CDR3
The CDRs are spatially separate in the primary sequence
They come together in the beta-sheets (Greek key motif) - this is known as the combining site
CDRs give rich diversity in the shape of the combining site
Ab:Ag are weak interactions so there needs to be a close fit over a large area with many interactions
Combining site redundancy
Occasionally, a small hydrophobic molecule may not occupy the full combining site despite having affinity to bind fully
This means these sites may be able to bind diverse epitopes (redundancy)
Allotype and idiotype in Ab diversity
Allotype – genetic variation between individuals for the same gene at the same loci (alleles)
Allotypic differences in Igs usually only involve 1 or 2 amino aicd changes which doest affect Ab:Ag binding
Idiotype – the uniqueness of an Ab’s combining site (somatic recombination)
Are unique and diverse in structure that, in theory, should be immunogenic in an animal of the same species
Anti-sera raised to this ‘idiotype’ can then block the biological activity
of this Ab through competitive interaction at the combining site
Antibody function
Agglutination – clumping cells together, precipitation and promotes phagocytosis (opsonisation)
Activation of other immune components
Classical complement pathway – triggers cell lysis
Phagocyotsis (opsonisation)
Activates natural killer cells (ADCC)
Passive immunity
Placenta
Breast feeding
Immobilisation of microbes
Also has an opsonising effect
Neutralisation of toxins
Binds to the regions on toxins that cause adverse biological effects
IgG antibodies are transferred over the placenta
IgA is passed to the baby through breastmilk
They can immobilise microbes – bacteria have flagella in order to move which makes it diffuclt for immune systems to catch them
Opsonising effect – causes phagocytes to ingest them
Neutralisation of toxins – snake venom
The antibody can bind to the toxin and prevent it from binding to its receptor neutralising the toxin
What are the characteristics of IgG
Two kappa or lambda L-chains and two gamma H-chains -Molecular weight 150KDa
Location – blood (plasma and serum), lymph, cerebrospinal fluid and peritoneal fluid
Subclasses -IgG1, IgG2, IgG3, and IgG4
Agglutination – IgG is a potent agglutinating agent and readily precipitates with multivalent Ag which helps to trigger phagocytosis
Passive immunity – all IgG except IgG2 pass through the placenta
Opsonisation – increases the likliehood of phagocytosis due to agglutination and precipitation
ADCC – antibody dependent cell cytotoxicity where FAB regions bind target and proturding Fc regions engage with the Fc gamma-receptor on cytotoxic immune cells (CD14 on natural killer cells)
Complement activation – is opsonising, chemotactic and triggers cell lysis
Neutralisation of toxins – binds to the active part of the toxin to prevent biological functions – horse IgG is given to humans to neutralise snake venom
Bacteria immobilisation – IgG interacts with epitopes on Ag (like the flagella) and prevents movement which leads to immobilisation then agglutination then phagocytosis
Neutralisation of viruses – binds to viral Ag on the viral coat and prevents attachment to host cells and therefore stops penetration
What are the characteristics of IgA
Two (kappa or lambda) L-chains and two alpha H-chains
160KDa per monomer
Exists as a dimer linked together by a J chain – mucosal IgA has a secretory component
Location – saliva, sweat, mucus, gastric juices (see slides)
Secretory IgA – formed during the transport through mucosa epithelial cells (transcytosis)
Dimeric IgA binds to poly Ig receptor on basolateral membrane of epithelial cell (endocyotsis)
At the apical surface (surface facing the lumen) the poly Ig receptor is cleaved by enzymes and releases IgA – this remains bound to the poly Ig receptor (secretory component) Summary of IgA function
Mucosal infections – primary Ig allotype in local bacteria /viral infections of the respiratory, intestinal and urinary tract
Bactericidal activity – IgA has no complement receptor but exhibits bactericidal activity on Gram-negative bacteria in the presence of a lysozyme secreted with the IgA
Anti-viral activity
Agglutination – crosslinks with Ag –> precipitation –> phagocytosis / lysis
What are the characteristics of IgM
2 kappa or lambda L-chains and 2 mew H-chains with a molecular weight of 900kDa
Exists as a pentamer linked together with the J chain
Mew H-chains have 4 C-domains (rather than 3 which can be seen in IgG and IgA) the c terminal of these C-domains are joined together by disulphide bonds
Location – 5-10% of total serum Ig in the blood – is expressed on the surface of B lymphocytes as a monomer and secreted by plasma cells as a pentamer
IgM is the only allotype synthesised by a foetus
Has a half-life of 5 days
Function summary
Agglutination – efficient due to the amount of FAB regions, valence is only 5 not 10 due to its rigid planar structure
Haemagglutination – recognises the Ag of erythrocytes – ABO blood groups
Complement activation – pentameric structure is a good complement fixer and activator – important for early bacteria defence
Can form macromolecular bridges between epitopes
What are the characteristics of IgD
2 kappa or lambda L-chains and 2 delta H-chains with a molecular weight of 180kDa
Location – blood (serum) in low amounts and on immature B lymphocytes
Half-life – 2.8 days, IgD is highly susceptible to protein degradation
It is expressed by B-lymphocytes and is “shed off” its surface – this is a sign of B-lymphocytes differentiating into plasma cells
Plasma cells do not secrete this allotype
What are the characteristics of IgE
2 kappa or lambda L-chains and two epsilon H-chains with a molecular weight of 200 kDa
Epsilon H-chains have 4 C-domains – rather than 3 seen with IgM
Location – found in the blood but in low concentrations
Binds to specific epsilon Fc receptors on mast cells and basophils
Has a half-life of 2 days in serum
Antibody functions
Low serum concentration does not account for its potent biological activity
Is the major Ig for parasites – massive activation of inflammatory response – anaphylactic shock
Fc receptors on cells bind to Fc region of IgE
IgE remains bound to mast cells and basophils for a long time – is involved in allergic responses
Does not mediate agglutination or activate the complement system
What is the primary antibody response
Latent or lag response
1-2 weeks in duration
T and B cells are challanged with an antigen – they proliferate (clonal expansion) - differentiate – detectable response (prescence of Ag specific Ig
Exponential production phase
Serum antibody concentration increases exponentially
Steady state
Balance between antibody production and degradation
Declining phase
What is the secondary antibody response
On further encounters with the same antigen
Shorter lag phase
Magnitude of antibody production is greater
Longer duration of Ig in serum (months-years)
What else occurs during the secondary antibody response
Shift in Ig class
IgG>IgM in serum
IgA and IgE may be produced and detected in serum
Maturation
Average increase in affinity of the antibody to the antigen
This is due to only the B cells with the epitope specific receptor on the surface will proliferate, differentiate and produce antibodies
Anamnestic (memory) - can last years and forms the basis of public health immunisation
