Antibody interaction with antigen Flashcards
What is an antigen?
molecule generate antibodies
what is an epitope?
the part of an antigen which is recognized by the antibody
Why can the epitope and paratope interact?
They are conformationally complimentary
How many hypervariable regions on each V region?
What is another name for the hypervariable regions?
3
Complementarity Determining Region?
Describe the hypervariable region
CDR loops interact with antigen
Poke out of the β-sheets of the Ig domain
How does binding happen?
Neither the antibody nor the antigen is changed by binding
One antibody binding site binds to one epitope on the antigen
The binding is non-covalent and is reversible
Describe antibody-antigen binding in terms of forces
Non-covalent interaction due to:
Ionic bonds
H-bonds
Hydrophobic bonds
Van der Waals forces
Lots of small forces add up to a strong interaction.
All critically dependent on the distance apart
Calculation of antibody affinity?
Each force acts over short distance
1x10-7mm or less.
Can be measured = affinity
What else is required for antibody production?
Additional signals are needed by the B cell once the BCR is ligated and Iga and Igb signal sent T cell help: cytokines
Additional co-stimulation e.g. CD19
Induces proliferation 1000-10000x
Differentiation into plasma cell
Functional consequences of Antibody binding
Describe Neutralisation (virus)
Abs may be important in limiting viral infectivity;
vaccine efficacy often assessed by measuring circulating, neutralising Abs
May inhibit virus-cell interaction, prevent endocytosis of virus or prevent uncoating inside endosome. More effective with complement
Describe Neutralisation (toxin)
Abs bind bacterial exotoxins: neutralise their effect by preventing attachment to cellular receptors (e.g. binding of cholera toxin to ganglioside GM1)
Stimulate toxin clearance from body (Fc-receptor mediated)
IgG and IgA are important neutralising antibodies
Describe Opsonisation
The coating (binding) of particles by either Ab, Complement or APP (eg CRP)
Ab bind microorganisms via the Fab and to cells by the Fc
Opsonisation increases the efficiency of the phagocytic process, allowing the organism to be cleared more effectively
Describe the Complement
Antibody independent ‘innate’ immunity
Classical and alternate pathways
Functions:
-Chemotaxis
-Opsonization
-Lysis of target cells
-Priming of the adaptive immune response.
describe Fc Receptors
How antibodies interact with cells
Fc Receptors, associate with g-chain
Activation occurs due to aggregation of receptors and signalling via Immunoreceptor Tyrosine-based Activation or Inhibition Motifs
Causes Antibody dependent cellular cytotoxicity, phagocytosis, apoptosis, mediator release and can enhance antigen presentation.
Describe FcgRI (CD64)
Binds monomeric IgG1 and IgG3 with high affinity and IgG4 with low affinity
No binding to IgG2
Expressed on mononuclear phagocytes
Involved in phagocytosis of immune complexes and mediator release
3 extracellular Ig domains
Associated with g-chain ITAM
Describe FcgRII (CD32)
FcgRIIa
Wide cellular distribution
Moderate affinity for monomeric IgG1 and IgG3.
High affinity for complexed IgG
Has ITAM
FcgRIIb
Same specificity for Ig but has ITIM!
Inhibition of Antibody Response
Inhibitory receptors contain an immunoreceptor
tyrosine inhibitory motif (ITIM) in their cytoplasmic
tails. These recruit inhibitory phosphatases, which
disrupt phosphorylation of activating receptors and
intracellular signaling molecules,
Describe FcgRIII (CD16)
FcgRIIIa
Transmembrane molecule with moderate affinity for monomeric IgG
Associated with g-, b-, z- h- chains of the CD3 complex as ITAMs
Expressed on monocytes, macrophages, NK cells and some T cells.
ADCC
FcgRIIIb
GPI linked with low affinity for monomeric IgG
Expressed on neutrophils and basophils
Activates by lipid raft formation and associates
Describe FcaRI (CD89)
Associated with g-chain
Expressed on myeloid cells
Can trigger phagocytosis, cell lysis and the release of inflammatory mediators
Binds both IgA1 and IgA2
Describe FceRI
Very high affinity receptor for IgE (1010L/mol)
Associates with g-chains and b-chain (abg2
receptor unit)
Expressed on mast cells and basophils.
Receptors always saturated (low serum IgE)
X-linking of these Ab molecules bound to FceRI
leads to mediator release e.g histamine
Describe FceRII (CD23)
Low affinity receptor for IgE
Expressed on leukocytes and lymphocytes.
Not a member of the Ig-Superfamily, similar to C-type lectins (eg mannose binding lectin)
CD23a Expressed on B cells and involved in IgE production
CD23bExpressed on lots of cell types and induced by IL-4
List and explain the two types of epitopes
Conformational epitope: conformation of epitope or 3D structure or epitope defines it function
Linear epitope: linear amino acid the sequence defines its function
Describe exogenous and endogenous antigens
Exogenous:
Origin - outside of the body, foreign antigens
APCs takes in exogenous Ag and processes it
APC presents it to T helper CD4+ T cell (MHC II)
Endogenous:
Origin - inside the body e.g. self antigen
Viral proteins produced inside our cells are endogenous in nature
No take up required
recognized by CD8+ T cell (MHC-I)
Calculation of antibody affinity
The affinity of an antibody is the sum of the attractive
and repulsive forces resulting from binding between the
paratope of a monovalent Fab fragment and its epitope.
This interaction will be reversible, so at equilibrium the
Law of Mass Action can be applied and an equilibrium
constant, K (the association constant), can be determined
What happens when a paratope-epitope interaction is broken?
In practice, due to the divalency of antibody
and multiple epitopes expressed on an antigen, large three dimensional complexes may be formed that do not readily dissociate; when a paratope–epitope interaction is broken both the antibody and antigen remain in proximity due to other paratope–epitope interactions, thus reassociation is favored.
