21 - antibodies Flashcards
structure of antibody (IgG) molecules
- 2 heavy (H) chains - 50kDa
- 2 light (L) chains - 25kDa
- variable (V) regions at N terminal - from antigen binding sites
- 2 binding sites per IgG
what are the 6 domain names of IgG?
C(H)1-3
C(L)
V(H)
V(L)
antigen binding sites
hypervariable (HV)
complementary-determining regions (CDR)
3 loops of HV on each H and L chains
6 CDR form the antigen binding sites
an immunoglobulin would have 12 in total
light chain domain structures
compact immunoglobulin domains - 100-110 AA
2 sheets of beta strands
form the antigen binding site
why are the domains important?
they are of structural and evolutionary significance
where are Ig-like domains found in other molecules?
- T cell receptors
- MHC molecules
- various cell adhesion molecules
- signalling molecules
what are the 2 types of light chains?
kappa or lambda
an antibody has either kappa or lama light chains - not one of each
no functional difference
what are the 5 classes of antibodies?
Ig: M, A, D, G, E
what are the 5 different heavy chain isotypes?
gamma mu delta alpha epsilon
differ in size, structure and function of C region
determine class of antibody
what is antibody valency?
the quantity of the antigen binding sites
single antibodies have a valency of 2
IgM pentamer has a valency of 10
IgM structure
- pentamer
- MW = 900,000
- covalent bound to J chain
- mainly found in blood
- valency of 10
- no hinge
- relatively low affinity
- can have avidity - total binding of the antibody
why is IgM mainly found in blood?
cannot cross cells and the placenta as its too larger
what is the first isotope to be produced by neonates and during a primary immune response?
IgM
functions of IgM
- complement activation
- agglutination
- B cell antigen receptor
- produced rapidly
- vital in primary response
- protects from common pathogens
IgG structure
- 4 domains - H(2)L(2)
- MW = 150,000
- most prevalent class in serum but not confined to intravascular compartment
- production increases in secondary response
- high affinity - good binder
- 4 sub-classes
why is IgG not confined to intravascular compartment?
can move across membranes and can cross the placenta from mother to neonate - helps to transfer immunity to child
what are the 4 subclasses of IgG
differences are all due to the hinge regions
IgG1 - 66%
IgG2 - 23%
IgG3 - 7%
IgG4 - 4%
longer hinge regions give the antibodies a longer reach
• doesn’t have much flexibility due to the disulphide bonds
functions of IgG
- predominant isotype in serum
- large increase in secondary response
- complement activation
- opsonisation
- ADCC
- only Ig to cross placenta
what does ADCC stand for?
antibody dependent cell cytotoxicity
antibody production in response to antigens
IgM first produced followed by IgG
if theres a 2nd exposure, memory B cells recognise the antigens and start the IgG response
• much larger IgG response - still a little IgM
IgA structure
- monomer in serum
- dimer in secretions
- MW = 70,000
- bound to J chain and secretory component alpha chain - 4 domains
- 2 sub-classes
what are the 2 subclasses of IgA?
differ in susceptibility to bacterial proteases
IgA1 - serum
IgA2 - secretions
what is the secretory component of IgA?
made by epithelial cells
involved in transport of IgA dimers through epithelial cells
if removed it cannot be secreted into the mucosal surface
transport of IgA across the epithelia
1) B cells secrete IgA
2) epithelial cells have Ig receptors specific to IgA which bind and engulf IgA
3) transported to the lumen where it is secreted
4) has to have binding portion cleaved
5) IgA and secretory component secreted in the lumen
what happens to IgA once transported to the gut lumen through epithelial cells?
IgA binds to the mucus layer overlying the epithelium
IgA in the gut neutralises pathogens and their toxins
functions of IgA
- inhibition of microbial adherence to mucosal surfaces
- neutralise pathogens and toxins
- prevent commensal bacteria entering blood stream
- present in early milk
what is commensal bacteria?
part of the normal flora in the mouth
IgD structure
- MW = 185,000
- 4 domains - long hinge
- low conc in serum
- can be present on B cells with IgM - only Ig’s that can be co-expressed
- present in upper respiratory tract
IgD functions
stimulate basophils and mast cells to release antimicrobial peptides - small proteins that kill pathogens
antigen receptor in B lymphocytes
IgE structure
- MW = 200,000
- 5 domains
- low conc in serum
- very potent effects
IgE functions
binds with high affinity to FcR on mast cells and basophils - release vasoactive/inflammatory mediators
role in allergy and asthma
importnat for ADCC
induces eosinophils and basophils to release histamine and proteases
what is anaphylaxis?
severe allergic reaction
• IgE
• Fc receptors in mast cells and basophils
• increased release of histamine
• increases TNF-alpha - inflammatory mediators
• increases vasodilation
what does an epipen do?
injects epinephrine
• increases vasoconstriction
• increases blood glucose levels
distribution of Ig in the body
- IgG and IgM predominant in blood
- IgG and monomeric IgA predominant in extracellular fluid
- dimeric IgA in secretions - breast milk
- foetus recipes IgG from mother
- brain has no Ig
class switching
happens post antigen stimulation
IgM can be switched to other Ig’s
irreversible DNA recombination - cutting and joining of heavy chain
once switched cannot switch back
affinity definition
strength of interaction between antigen binding site and epitope
valency definition
all Ig’s have a valency of 2 but some are dimers or pentameric so have a higher number
avidity definition
measurement of overall strength of the antibody-antigen interactions
it depends on:
• affinity
• valency
• structural parts between the 2 entities interacting
