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Immunoglobulins:
Structure and Function
Immunoglobulins:Structure and
Function
Amount of serum
protein
• Definition: Glycoprotein molecules that are
present on B-cells (BCR) or produced by
plasma cells (antibodies) in response to an
immunogen
+
albumin
Separation of serum
proteins depending on
their charge, size,
molecular weight
globulins
α1
α2
β
γ
Electroforetic mobility
Immune serum
Ag adsorbed serum
Immunoglobulin Structure
• Heavy & Light
Chains
• Disulfide bonds
– Inter-chain
– Intra-chain
Disulfide
bond
Carbohydrate
CL
VL
CH1
VH
CH2
Hinge Region
CH3
Immunoglobulin Fragments:
Structure/Function Relationships
Ag Binding
Complement Binding Site
Binding to Fc
Receptors
Placental Transfer
Immunoglobulin Structure
• Variable &
Constant
Regions
• Hinge Region
• Domains
– VL & CL
– VH & CH1 - CH3
(or CH4 in IgM)
• Oligosaccharides
Disulfide
bond
Carbohydrate
CL
CH1
VH
CH2
Hinge Region
CH3
Ribbon structure of IgG
mIg = BCR
HYPERVARIABLE REGIONS
Immunoglobulin Fragments:
Structure/Function Relationships
• Fab
– Ag binding
– Valence = 1
– Specificity
determined by VH
and VL
Papain
VH
VL
• Fc
– Effector
functions
Fc
Fab
Immunoglobulin Fragments:
Structure/Function Relationships
Pepsin
• F(ab’)2
– Antigen binding:
valence=2, bivalence!
Fc
Peptides
F(ab’)2
Why do antibodies need an Fc region?
The (Fab)2 fragment can Detect antigen
•
Precipitate antigen
•
Block the active sites of toxins or pathogen-associated
molecules
•
Block interactions between host and pathogen-associated
molecules
but can not activate
•
Inflammatory and effector functions associated with cells
•
Inflammatory and effector functions of complement
•
The trafficking of antigens into the antigen processing
pathways
NEUTRALISATION
•
Immunoglobulin Structure-Function Relationship
• Cell surface antigen receptor on B cells (BCR)
Allows B cells to sense their antigenic environment
Connects extracellular space with intracellular signalling
machinery
• Secreted antibody
Neutralisation
Arming/recruiting effector cells
Complement fixation
Variability in different regions of the Ig
determines Ig classes or specificity
isotype
allotype
(Classes/subclasses)
Allelic variants
Sequence variability of H/Lchain constant regions
IgG – Gm allels
Ig classes: IgG, IgA, IgE, IgD, IgM;
subclasses: IgA1-2, IgG1-4
(light chain izotypes: κ, λ)
idiotype
Sequence variability of H
and L-chain variable
regions (individual, clonespecific)
responsible for antigen
specificity
Human Immunoglobulin Classes
•
•
•
•
•
IgG - Gamma (γ) heavy chains
IgM - Mu (μ) heavy chains
IgA - Alpha (α) heavy chains
IgD - Delta (δ) heavy chains
IgE - Epsilon (ε) heavy chains
Light Chain Types
• Kappa (κ)
• Lambda (λ)
Izotypes!
encoded by different structural gene segments (isotypes)
Ig isotype
Serum
concentration
Characteristics, functions
Trace
amounts
 Major isotype of secondary
(memory) immune response
 Complexed with antigen activates
effector functions (Fc-receptor
binding, complement activation
 The first isotype in B-lymphocyte
membrane
 Function in serum is not known
Trace
amounts
 Major isotype in protection against
parasites
 Mediator of allergic reactions (binds
to basophils and mast cells)
3-3,5 mg/ml
 Major isotype of secretions (saliva,
tear, milk)
 Protection of mucosal surfaces
12-14 mg/ml
1-2 mg/ml
 Major isotype of primary immune
responses
 Complexed with antigen activates
complement
 Agglutinates microbes
 The monomeric form is expressed in
B-lymphocyte membrane as antigen
binding receptor
Free pentameric IgM
structure
(”star-shape”)
IgM binding to an antigen
(”crab-shape”)
Different antibody isotypes
Characterization of antibody-antigen binding
Valence: the number of
bonds that a given
antibody can form with
one or more antigens
Affinity: strength of a
single bond between a
given antigen and a
given antibody
Avidity: in case of an
antibody, it means the
combined strength of
multiple bonds
IG ISOTYPE PRODUCTION OVER THE
ONTOGENESIS
BEFORE BIRTH
AFTER BIRTH
Breast milk
IgA
IgM
100%
(ADULT)
Maternal IgG
IgG
IgA
0
3
6
MONTHS
9
12
4 5
ADULT
YEARS
IgA in breast milk has an important bridge role!
Secretory IgA and transcytosis
S
S
SS
SS
SS
SS
ss
ss
J
J
S
S
S
S
S
S
S
S
S
S
S
S
B
J
ss
SS
Epithelial
cell
pIgR & IgA are
internalised
ss
SS
S
S
J
S
S
J
SS
SS
S
S
ss
IgA and pIgR
are transported
to the apical
surface in
vesicles
SS
‘Stalk’ of the pIgR is degraded to release
IgA containing part of the pIgR - the
secretory component
SS
B cells located in the submucosa
produce dimeric IgA
Polymeric Ig
receptors are
expressed on the
basolateral surface
of epithelial cells to
capture IgA
produced in the
mucosa
EFFECTOR FUNCTIONS OF
ANTIBODIES
1)
2)
3)
4)
Neutralisation
Opsonization (facilitated phagocytosis)
ADCC
Complement activation (see it later)
NEUTRALISATION
OPSONIZATION
An opsonin is any molecule that enhances phagocytosis by marking an
antigen for an immune response. In the picture opsonins are antibodies.
ANTIBODY DEPENDENT CELLULAR CYTOTOXICITY
(ADCC)
Ig. Concentration
ANTIBODY
PRODUCTION DURING IMMUNE RESPONSE
Immunoglobulin
concentration
Szekunder
’lasyecondary
Secondary
response response
Primary
primerresponse
response
IgG
IgA
IgE
IgM
IgM
Primary response
against the B
antigen
5
„A” antigen
„A” antig é n
10
15
20
25
30
„A” and „B” antigen
„A” és „B”
antigén
napok
days
Polyclonal antibody response
Ag
Immunserum
Polyclonal
antibody
Set of B-cells
Ag
Activated B-cells
Antibodyproducing
plasma-cells
Antigen-specific
antibodies
I. Passive
Methods of immunisation
Serum containing
the specific
antibody
(usually IgG)
Endagered subject
- Does not depend on the
immune response of the
recipient
- Acts immediately
- Short-term protection
only (elimination of Ig’s!)
The subject with
specific antibody
II. Active immunisation
Vaccination is a good example, when not antibodies but inactivated
or attenuated pathogens or purified antigens from pathogens are
administered sc. Immune response depends on the immune state o
the recipient, immune protection needs time to develop, but long
term protection is provided (memory cells).
Intravenous immunoglobulin
PASSZÍV IMMUNIZÁLÁS
Pooled intravenous immunoglobulin (IVIg)
(Intratect, Intraglobin, Octagam, Gammagard)
(approx. 59% IgG1, 36% IgG2, 3% IgG3, 2%
IgG4 and maximally 5% IgA)
PROTECTED
SUBJECTS
serum antibodies
ENDANGERED
SUBJECT
No activation of the immune system
Acts immediately
The protection is short-term only
Elimination of Immunoglobulins
Intravenous immunoglobulin #1
Low dose: passive immunisation
Indications: primary or secundary immune deficiency
- congenital agammaglobulinaemia
- severe combined immune deficiency (SCID)
- Wiskott-Aldrich syndrome
- multiplex myeloma or chronic lymphoid leukemia
- premature babies
- allogenic bone marrow transplantation
- congenital HIV-infection (AIDS)
Intravenous immunoglobulin #2
High dose: immune suppression
The „physiologic” immunsuppressive agent!
Especially useful in the autoimmune diseases of
children, the only limit is the price.
Indications:
- immune thrombocytopenia (ITP)
- dermatomyositis/polymyositis
- myasthenic crisis (myasthenia gravis)
- Guillain-Barré syndrome
- graft versus host reaction (after transplantation)