Antigen recognition is mediated by specific antigen receptors

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Antigen recognition is mediated by specific
antigen receptors

• Recognition is a central feature of the immune
  response, required for detection and elimination
  of dangerous organisms (pathogens).
• This recognition is mediated by specific antigen
  receptors which bind to structures (antigens) on,
  or derived from, pathogens.
• Antigen receptors can be secreted or
  cell-associated.
• Antigen?any structure which elicits an immune
  response

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Receptors for antigen in the immune system

• Signalling receptors
• B-cell (immunoglobulin)
• T-cell receptors
• Substantial somatic diversification of receptors
  within the individual
• Antigen display receptors
• MHC Class I and II
• Limited diversity, all of which is inherited as
  part of polymorphism in the species

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Immunoglobulin properties

• Need to study homogeneous, pure protein
• Myelomas-tumours of plasma cells
• Monoclonal antibodies

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HYBRIDOMAS TO MAKE MONOCLONAL ANTIBODIES
LYMPHOCYTE
HYBRIDOMA
MYELOMA
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A prototype antibody molecule
Light chain
Disulfide bonds
Heavy chain
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ENZYME DIGESTION OF IMMUNOGLOBULINS
Fragment antigen-binding
Fragment crystallisable
Papain
Pepsin
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IMMUNOGLOBULIN-THE BASIC UNIT
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Structure of an immunoglobulin molecule (2)
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Hypervariable regions in the antibody
sequence Complentarity determining regions
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A V-REGION
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ANTIBODY FUNCTION
ANTIGEN BINDING

CDRs
Complementarity determining regions
EFFECTOR FUNCTION (business end)
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ANTIBODY CLASSES AND SUBCLASSES (ISOTYPES)

• CLASSES
• IgM, IgG, IgD, IgA, IgE
• SUBCLASSES
• IgG1, IgG2, IgG3, IgG4

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IMMUNOGLOBULIN CLASSES-THE BASIC SUBUNITS
MONOMER PENTAMER MONOMER
DIMER MONOMER
MONOMER WHEN ACTING
AS B-CELL RECEPTOR FOR ANTIGEN
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THE PENTAMERIC IgM MOLECULE
FIRST LINE OF DEFENCE LOW AFFINITY-HIGH AVIDITY
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DIMERIC IgA
Protection of the mucosa
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AFFINITY AND AVIDITY
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A typical antibody molecule
Antigen binding region
Fc region interacts with innate effector
mechanisms and greatly amplifies them
Fc region
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Fc Receptors

• High affinity-Ig pre-bound to receptor
• Waits for antigen to come along.
• IgG, IgE
• Low affinity- receptor not normally occupied.
• only occupied after antibody has become complexed
  to antigen (useful in phagocytosis)
• IgG, IgA, IgE

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Role for IgE on mast cells and basophils
High affinity receptor for IgE
antigen
Antigen comes to the mast cell which already has
IgE attached to its receptor
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Role for IgE and eosinophils
eosinophil
Low affinity receptor
antigen
Antibody coated antigen comes to the cell
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Immunoglobulins-properties
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Immunoglobulins-properties
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Protective effects of antibody
Opsonisation and antibody-dependent destruction
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ANTIBODY CAN BLOCK BINDING AND ENTRY OF VIRUSES
INTO CELLS
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Interaction between innate and adaptive immunity.
The complement system
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COMPLEMENT ACTIVATION ON A CELL SURFACE
INFLAMMATORY MEDIATORS CHEMOTACTIC
FACTORS OPSONISATION CELL LYSIS
C5 C6 C7 C8 C9
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Uses of Monoclonal Antibodies

• Diagnostic agents (histology, immunoassays)
• Experimental probes for cell biology
• Therapeutic agents
• What are the advantages over polyclonal
  antibodies raised by immunisation of larger
  animals?

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Therapeutic Monoclonal antibodies for killing
lymphocytes
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CD52 is strongly expressed on lymphocytes and not
on blood stem cells
CD52
lymphocytes
STEM CELLS
MYELOID CELLS
PLATELETS
RED CELLS
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ANTIBODY FUNCTION
ANTIGEN BINDING

CDRs
Complementarity determining regions
EFFECTOR FUNCTION (business end)
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HUMANISATION OF ALEMTUZUMAB (ANTIBODY ENGINEERING)
ANTIGEN BINDING
CDRs
EFFECTOR FUNCTION (business end)
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ALEMTUZUMAB-THE FIRST HUMANISED THERAPEUTIC
ANTIBODY
Riechmann, L. Clark, M., Waldmann, H., Winter,
G. 1988 Reshaping human antibodies for therapy.
Nature 332, 323-327.
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The first administration of humanised CAMPATH-1H
(ALEMTUZUMAB)
Hale, G. Clark, M.R., Marcus, R., Winter, G.,
Dyer, M.J.S., Phillips, J.M., Riechmann, L.
Waldmann, H. 1988 Remission Induction in
Non-hodgkin Lymphoma with Reshaped Human
Monoclonal Antibody CAMPATH-1H Lancet II,
1394-1399
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90 reduction in relapse rate after Alemtuzumab.
Alemtuzumab treatments
Months before and after Alemtuzumab
Correct as of 31 December 2003
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ANTIBODIES

• RECEPTORS FOR ANTIGEN ON B-CELLS
• WHEN SECRETED IMMUNOGLOBULIN ANTIBODIES
• ONE B-CELL MAKES ONLY ONE ANTIBODY
• HYBRIDOMAS THROUGH CELL FUSION
• MONOCLONAL ANTIOBODIES
• ANTIBODY ENGINEERING
• THERAPEUTICS AND DIAGNOSTICS
• EXPERIMENTAL PROBES