Antibody Classes are defined by the constant region of

1
Antibody Classes are defined by the constant
region of heavy chain.
IgE (?)
VH
IgM (?)
VH
0.004
VH
VH
VL
VL
VL
10
C?1
C?1
VL
C?1
C?1
CL
CL
CL
CL
C?2
C?2
C?2
C?2
C?2 and C?2 replace hinge region.
C?3
C?3
C?3
C?3
C?4
C?4
C?4
C?4
IgG (?) IgG1, IgG2, IgG3, IgG4
IgA (?) IgA1, IgA2
IgD (?)
VH
0.2
75
VH
VH
VH
15
VH
VH
VL
VL
VL
VL
C?1
C?1
C?1
VL
C?1
VL
C?1
C?1
CL
CL
CL
CL
CL
CL
C?2
C?2
C?2
C?2
C?2
C?2
C?3
C?3
C?3
C?3
C?3
C?3
2
IgM is the most efficient activator of complement.
monomer
C1q
C1q needs to bind at least two Fc regions
simultaneously in order to be activated.
Complement activation
IgM gt IgG1, IgG3 gt IgG2
IgG4, IgA, IgE, IgD do not activate complement.
One IgM pentamer can activate C1.
High concentrations of IgG are needed to activate
C1.
3
Antibody only activates C1 after binding to
antigen.
C1q binding site in C?3 is revealed.
Antigen bound IgM
Free IgM
Free IgM
C?3
IgM bound to antigen
4
IgG and IgA can serve as opsonin for phagocytes.
IgG1 and IgG3
IgA
Fc?RIIIB
Fc?RI
Fc?RIIA
Fc?R
Fc?RIV
?
?
?
?
?2
?2
?2
? subunit determines Ig class specificity.
?2 subunit mediates signal transduction.
FC?RIIIB
C?2-B
C?2-A
5
Fc?RII and III only bind antibody in immune
complexes.
Free IgG
bacteria
phagocyte
phagocyte
Stable high avidity interaction between multiple
antibodies and Fc?RII and III
Low affinity interaction between single antibody
and Fc?RII and III
activation
6
Cross-linking of FcR leads to activation.
?
FcR
?
?
?2
?2
?2
ITAM (Immunoreceptor Tyrosine- based Activating
Motif)
19-amino acid motif with 2 conserved tyrosines (Y)
bacteria
Cross-linking (clustering) of FcR by immune
complex
?
?
?
?2
?2
?2
Y
lyn
lyn
lyn
Y
Activation of Src family of tyrosine kinases
(lyn, lck)
Lyn or lck phosphorylate the tyrosines in ITAM.
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Ca2
Calcium influx induced by Ca2 release from ER
Ca2
Y
ITAM
Syk
Y
Phosphorylation and activation of
phospholipase-C???PLC??
Syk binds to phosphorylated ITAM through SH2
domains. Syk is activated.
(inositol triphosphate)
PI(4,5)P3
IP3
Phosphatidylinositol-4,5-biphosphate
Ca2
DAG (diacylglycerol)
Ca2 and DAG activates downstream factors
Ca2 store
Endoplasmic reticuluum
Phagocytosis, inflammation, respiratory burst
8
Analysis of Fc?RIII function
Red blood cell opsonized with IgG1, IgG2a,
IgG2b (mouse IgG subclasses are different from
human IgG)

Macrophage
phagocytosis
Phagocytosis of red blood cells
Fc?RIII/
Fc?RIII-/-
Phagocytosis through IgG1 is defective.
Phagocytosis through IgG2a and IgG2b is
normal. Different classes of IgG use different
Fc?Rs.
9
Fc?RIII mediates mast cell activation.
Ovalbumin (ova)
Immune complex of IgG and ova
Mast cell activation
Cytokines (IL-1, TNF-?)
degranulation
histamine, serotonin
Lipid mediators (Prostaglandins, leukotrienes)
10
Immune complex-mediated inflammation (Arthurs
reaction, type III hypersensitivity)
Complements are not major contributors of immune
complex-induced inflammation.
Fc?RIII-/-
Inject IgG against ovalbumin
Ear skin
IgG-immune complex
Mast cell activation through Fc?R
Fc?RIII/
inflammation
Inject blue dye and ovalbumin intravenously 2
hours later
Local accumulation of blue dye
Increased vascular permeability
11
Complements facilitate the removal of immune
complexes.
Systemic lupus erythematosus (SLE)
High levels of immune complexes between
autoantibodies and self- antigens
Immune complexes are induce inflammation,
particular in kidney and the joint.
Deficiency in C1 and C4 can cause SLE.
12
Inhibitory Fc?R (Fc?RIIB)
Activating receptors
Inhibitory receptor
FC?RIIIB
FC?RI
FC?RIIA
Fc?RIIB
Fc?RIV
?
ITIM
?
?
ITAM
?2
?2
Immunoreceptor Associated Inhibitory Motif
13-amino acid sequence with a conserved tyrosine
(Y)
13
Inhibitory receptor reduces the activation of
PLC-??
Y
ITAM
Syk
Y
ITIM
Y
SHIP
phosphatase
Cascades of phosphorylation
Phosphorylation and activation of PLC-?
PLC-?
PI(4,5)P2
IP3
DAG
14
Immune complex causes lung injury through Fc?R.
Mice
Intravenous injection of Ovalbumin
Introduce anti-ovalbumin IgG into the lung
Hemoharrge
IgG-Ova immune complex induces inflammation
(Arthur reaction in the lung)
Neutrophil infiltration
Lack of ? chain of Fc?R protects immune
complex-induced inflammation.
Complements are not major contributors to immune
complex-induced inflammation.
15
Loss of inhibitory Fc?RIIB increases inflammation.
16
Increased Ca2 influx in the absence of Fc?RIIB
Enhanced inflammatory cytokine production in the
absence of Fc?RIIB
Enhanced phagocytosis of IgG opsonised RBC in the
absence of Fc?RIIB
17
IgG isotypes have different activities on
activating and inhibitory receptors
Mouse IgG isotypes (IgG1, IgG2a, IgG2b, IgG3)
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(No Transcript)
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IgA is secreted to the mucosal surface to
inhibit the attachment of pathogens.
IgM is also secreted through the same mechanism.
transcytosis
20
Neutralization of toxins
IgG is produced later during B cell activation,
and generally has higher affinity toward antigens.
High affinity IgG is the main protection against
toxins.
21
Maternal antibodies protect the baby.
Maternal IgG is transported by Fc?Rn (neonatal
receptor) across placental to the fetus.
Maternal IgA is in breast milk.
22
FcRn is homologous to Class I MHC.
Different mode of binding to Fc region of IgG
compared with Fc?R
Fc?RIII
Ig Superfamily
23
Blood type difference between mother and fetus
ABO blood type
ABO antigens are carbohydrates on red blood cells.
Antibodies against A and B antigens are induced
against intestinal bacterial antigens.
Blood transfusion reaction (Type II
Hypersensitivity)
When mother and fetus have different ABO blood
types, no lysis of red blood cells happens.
Anti-A, B antibodies are IgM, and are not
transported across the placenta.
B type blood
Anti-A antibodies
RBC (A type)
lysis
complement
24
Hemolytic disease of the newborn (erythroblastosis
fetalis)
13 women are Rh negative.
Anti-Rh IgG activates macrophages through FcRs.
25
Antibody functions
Complement activation
Phagocytosis (neutrophils, macrophages)
Oxidative burst (neutrophils, macrophages)
Activate effector cells
Inflammation (mast cells, macrophage,
neutrophils)
Cell-mediated toxicity (natural killer cells)
Prevent entry of pathogens
Neutralization
Neutralize toxins
Maternal protection of baby
26
Questions
What determines antibody class?
What are the functional differences between
antibody classes?
Which classes of antibodies activate complements?
Which classes of antibodies can serve as opsonins
for phagocytosis?
How do immune complexes induce inflammation?
How are immune complexes cleared?
What are activating and inhibitory Fc receptors?
How do mothers provide immnue protection for the
babies?
What are the causes of blood transfusion
reactions and hemolytic diseases of the newborn?
Relevant parts in textbook
P93-100, p102-105, p170-171, p185-186, p388-393