2 antibody antigen interactions

Question 1

what part of the antigen does the antibody recognise?

Answer 1

The epitope
(the complimentary part of the antibody is the paratope)

the antibody doesn’t necessarily see a continuous piece of protein, rather could be several discountinuous segments linked together brought about by the tertiary structure of an antigen

Question 2

antibody structure - variable region

Answer 2

Each V region has 3 hypervariable regions.

The light and heavy chain regions have 6 hypervariable regions which constitute the Complimentarity Determining Region (CDR)

This is where the antibody and antigen interact.

CDR loops interact with the antigen. These CDRs have a massive potential for diversity which gives the wide range of antigen specificities - the CDRs poke out of the B-sheets of the Ig domain.

Question 3

How does the binding happen?

Answer 3

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 4

Forces Involved in Antigen-Antibody Interactions

Answer 4

H-bonds; ionic bonds (electrostatic); Hydrophobic bonds; Van der Waals forces

• Lots of small forced add up to a strong interaction - all critically dependent on the distance apart
• Each force acts over v. short distance (can be measured = affinity)

Question 5

affinity vs avidity

Answer 5

affinity gives the strength of interaction between an antibody and its epitope

More important is the avidity of that interaction, the sum of the individual affinities

Monovalent interaction – low avidity
Polyvalent - high

Functionally avidity is likely to be more important than affinity

Question 6

What else is required for antibody production?

Answer 6

Additional signals needed by the B cell once BCR ligated and Igα and β signal sent

T cell help: cytokines

Additional co-stimulation e.g. CD19

Induces proliferation 1000-10000x

Differentiation into plasma cell

Question 7

Functional consequences of antibody binding

Answer 7

• neutralisation of microbes and toxins
• opsonisation and phagocytosis
• ADCC (NK cell)
• complement activation
• microbe lysis
• inflammation

Question 8

Neutralization (virus)

Answer 8

• Depends on type of virus, target cell & class of Ab
• Abs may be important in limiting viral infectivity; vaccine efficacy often assessed by measuring circulating, neutralizing Abs
• May inhibit virus-cell interaction, prevent endocytosis of virus or prevent uncoating inside endosome. More effective with complement

Question 9

Neutralization (toxin)

Answer 9

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)

Question 10

Opsonization

Answer 10

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

Opsonization increases the efficiency of the phagocytic process, allowing the organism to be cleared more effectively

Question 11

Complement

Answer 11

Antibody independent ‘innate’ immunity

Classical and alternate pathways

Functions:

• Chemotaxis
• Opsonization
• Lysis of target cells
• Priming of the adaptive Immune Response.

Question 12

Fcγ receptors

Answer 12

These receptors are how Ab interact with cells

• Fc Receptors, associate with γ-chain
• Activation occurs due to aggregation of receptors
• A lot of the Fc receptors are associated with signal transduction pathways, and some have their own activation motifs on their intracellular tail
• Immunoreceptor Tyrosine-based Activation (Inhibition) Motifs
• Causes ADCC, phagocytosis, apoptosis, mediator release and can enhance antigen presentation.

Question 13

FcγRI (CD64)

Answer 13

Binds monomeric IgG1 and IgG3 with high affinity

Binds 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 γ-chain ITAM

Question 14

FcγRII (CD32)

Answer 14

2 forms a and b

FcγRIIa:
Wide cellular distribution
Moderate affinity for monomeric IgG1 and IgG3.
High affinity for complexed IgG
Has ITAM

Doesn’t need to be associated with other signal transduction molecules

FcγRIIb:
Same specificity for Ig BUT… Has an ITIM!

ITIM involved in inhibitory processes to stop an immune response

Question 15

Inhibition of Ab response

Answer 15

Negative feedback on the process- concentration dependent

Question 16

FcγRIII (CD16)

Answer 16

Expressed on NK cells and some T cells
2 forms
FcγRIIIa is a transmembrane molecule (surface bound – transduction function on NK cells)

FcγRIIIb is glycosyl phosphatidyl inositol (GPI) linked. Expressed on granulocytes

Question 17

FcγRIIIa

Answer 17

Moderate affinity for monomeric IgG

Associated with γ-chain, β chain or ζ −η chains of the CD3 complex as ITAMs

Expressed on monocytes, macrophages, NK cells and some T cells.

Question 18

FcγRIIIb

Answer 18

Low affinity for monomeric IgG
Expressed on neutrophils and basophils

Activates by lipid raft formation and associates with other membrane signalling molecules

Question 19

FcαRI (CD89)

Answer 19

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

Question 20

FcεR

Answer 20

FcεRI
-high affinity receptor on Mast cells and basophils. Associates with γ-chains and β-chain (αβγ2 receptor unit)
-Very high affinity for IgE (1010L/mol)
Receptors always saturated (low serum IgE)
X-linking of these Ab molecules bound to FceRI leads to mediator release eg Histamine

FcεRII
-low affinity receptor expressed on leukocytes and lymphocytes
-Not a member of the Ig-Superfamily, similar to C-type lectins (eg Mannose binding lectin)
-CD23a and CD23b
CD23a: expressed on B cells and involved in IgE production
CD23b expressed on lots of cell types and induced by IL-4