Antibody interaction with antigen

Question 1

What is an antigen?

Answer 1

molecule generate antibodies

Question 2

what is an epitope?

Answer 2

the part of an antigen which is recognized by the antibody

Question 3

Why can the epitope and paratope interact?

Answer 3

They are conformationally complimentary

Question 4

How many hypervariable regions on each V region?
What is another name for the hypervariable regions?

Answer 4

3
Complementarity Determining Region?

Question 5

Describe the hypervariable region

Answer 5

CDR loops interact with antigen
Poke out of the β-sheets of the Ig domain

Question 6

How does binding happen?

Answer 6

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

Question 7

Describe antibody-antigen binding in terms of forces

Answer 7

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

Question 8

Calculation of antibody affinity?

Answer 8

Each force acts over short distance
1x10-7mm or less.
Can be measured = affinity

Question 9

What else is required for antibody production?

Answer 9

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

Question 10

Functional consequences of Antibody binding

Answer 10

Question 11

Describe Neutralisation (virus)

Answer 11

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

Question 12

Describe Neutralisation (toxin)

Answer 12

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

Question 13

Describe Opsonisation

Answer 13

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

Question 14

Describe the Complement

Answer 14

Antibody independent ‘innate’ immunity
Classical and alternate pathways
Functions:
-Chemotaxis
-Opsonization
-Lysis of target cells
-Priming of the adaptive immune response.

Question 15

describe Fc Receptors

Answer 15

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.

Question 16

Describe FcgRI (CD64)

Answer 16

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

Question 17

Describe FcgRII (CD32)

Answer 17

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!

Question 18

Inhibition of Antibody Response

Answer 18

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,

Question 19

Describe FcgRIII (CD16)

Answer 19

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

Question 20

Describe FcaRI (CD89)

Answer 20

Associated with g-chain
Expressed on myeloid cells
Can trigger phagocytosis, cell lysis and the release of inflammatory mediators
Binds both IgA1 and IgA2

Question 21

Describe FceRI

Answer 21

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

Question 22

Describe FceRII (CD23)

Answer 22

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

Question 23

List and explain the two types of epitopes

Answer 23

Conformational epitope: conformation of epitope or 3D structure or epitope defines it function
Linear epitope: linear amino acid the sequence defines its function

Question 24

Describe exogenous and endogenous antigens

Answer 24

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)

Question 25

Calculation of antibody affinity

Answer 25

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

Question 26

What happens when a paratope-epitope interaction is broken?

Answer 26

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.