Transcript Genetic Disorders

Immunoglobulin
Structure and Function
Lecture 8
Dr. Nabil MTIRAOUI, M.Sc, Ph.D
Definition and Properties
Outline of Lecture
1. Recognize
the
structure
of
immunoglobulin molecule
2. Know the different types of
immunoglobulin.
3. Understand the biological activities
of each type.
4. Differentiate between types of
immunoglobulin.
5. Define isotype switching and
antibody diversity.
6. Differentiate between types of FC
receptors.
Antibodies (or Immunoglubulins)

The chemical information of immunoglobulin was provided
by Tiselius and Kabat in the early 1940s.

In 1950s, Porter and Edelman revealed the basic structure of
immunoglobulin molecule.

Antibodies
are
products
of
antigen-
activated
lymphocytes.

They are the main effectors of humoral immunity.
B-
Definition
Amount of protein
+
albumin
globulins


1
2


Immune serum
Ag adsorbed serum
Mobility


An antibody or immunoglobulin (Ig) is a glycoprotein that is made by
plasma cells in response to an antigen and can recognize and bind to
the antigen that caused its production.
Antibodies are produced by B cell
General Functions of Immunoglobulins

Ag binding
 Can
result in protection
 Valency

Effector functions
 Fixation
of complement
 Binding
to various cells
 Usually
requires Ag binding
Basic Immunoglobulin Structure
1.
2.
3.
4.
5.
6.
Fab region
Fc region
Heavy chain with one variable (VH) domain
followed by a constant domain (CH1), a hinge region, and two
more constant (CH2 and CH3) domains.
Light chain with one variable (VL) and one constant (CL)
domain
Antigen binding site (paratope): It is the area of Ig molecules
that interacts specifically with epitope of the Ag
Hinge regions.
Basic Immunoglobulin Structure




Abs have more than one antigen
combining site Some bivalent Ab
molecules can combine to form
multimeric Abs that have up to 10
combining sites.
All immunoglobulin have a basic
structure composed of 4 polypeptide
chains connected to each other by
disulfide bonds.
Each light chain consist of 220 Aa
and has a mass of approx. 25kDa.
Each heavy chain consists of about
440 Aa and has a mass of 5070kDa.
Basic Immunoglobulin Structure





Both light and heavy chains contain
2 different regions
constant and variable region
The four chains are arranged in the
form of a flexible “Y” with the
hinge region and is termed as
crystallizable fragment (Fc) and
contains the site at which Ab binds.
Top of the “Y” consist of two Ag
binding fragments (Fab) that bind
with antigenic determinant sites.
The four chains are linked by
disulfide bonds.
Basic Immunoglobulin Structure
Light chain:
 The light chain may be either
of two distinct forms called
“Kappa” and “Lambda” and
can be distinguished by aa
sequence of carboxyl portion
of the chain.
Heavy chain:
 In
the heavy chain NH2
terminal has a pattern of
variability similar to that of
kappa and lambda of the light
chain.
The variable (V) regions.



The first 100 or so amino acids at the N-terminal of both H and L
chains vary greatly from antibody to antibody.
These are the variable (V) regions.
 The amino acid sequence variability in the V regions is especially
pronounced in 3 hypervariable regions.
 Together they construct the antigen binding site against which the
epitope fits
Only a few different amino acid sequences are found in the Cterminals of H and L chains.
These are the constant (C) regions.
The variable (V) regions.
Variability Index
HVR3
15
0
10
0
5
0
0
HVR2
HVR1
FR2
FR1
25
50
Amino acid residue
FR3
7
5
FR4
10
0
The Hypervariable regions.

Hypervariable regions: In the variable regions of
both
L&H
chains,
there
are
3
extremely
hypervariable amino acids sequences that form the
Ag binding sites.

The
hypervariable
regions
form
the
region
complementary in structure to the epitope. These
regions are involved in the formation of paratope.
The constant (C) regions.
 two
different kinds of C regions for
their L chains producing
 kappa (κ) L chains
 lambda (λ) L chains
 five different kinds of C regions for
their H chains producing
 mu (µ) chains (the H chain of IgM
antibodies)
 gamma (γ) chains (IgG)
 alpha (α) chains (IgA)
 delta (δ) chains (IgD)
 epsilon (ε) chains (IgE)
Fc and Fab regions

The proteolytic enzyme papain breaks each Ig molecule into
3 fragments at the hinge region.

The single crystallizable fragment (Fc region) includes part
of the constant domain that occupies the stem.

There are 2 antigen-binding fragments (Fab region), which
include the entire light chain and variable and constant
portions of the heavy chain.

Ig G
Papain
2 Fab + Fc
Fc and Fab regions








Antigen-binding fragment (Fab)
recognize Ag
contain Ig idiotype
Unique protein sequence that
identifies each Ab
Crystallizable fragment (Fc)
define isotype of Ig
bind FcR for all functional attributes
of Ab
Fab link to Fc by hinge region
Fonction of Fc and Fab regions



By binding to specific proteins the Fc region ensures that
each antibody generates an appropriate immune response
for a given antigen.
The Fc region also binds to various cell receptors, such as
Fc receptors, and other immune molecules, such as
complement proteins.
Thus, Ab mediates different physiological effects including
opsonization, cell lysis, and degranulation of mast cells,
basophils and eosinophils.
Activation of complement




Antibodies that bind to surface antigens on, for example a
bacterium, attract the first component of the complement
cascade with their Fc region and initiate activation of the
"classical" complement system
This results in the killing of bacteria in two ways;
First, the binding of the antibody and complement
molecules marks the microbe for ingestion by phagocytes in
a process called opsonization;
Secondly, some complement system components form a
membrane attack complex to assist antibodies to kill the
bacterium directly.
Types of FC receptors
FC receptor
Affinity to Ig
Cell distibution
Function
FcγRl (CD64)
High binds IgG1a
and IgG3
MQ. Neutrophils
and eosinophils
Phagocytosis,
activation of
phagocytes
FcγRllA (CD32)
Low
MQ. Neutrophils
and
eosinophils,platelet
Phagocytosis,cell
activation
(inefficient)
FcγRllB (CD32)
Low
B lymphocytes
Feed back
inhibition of B cells
FcγRlllA (CD16)
Low
NK
ADCC
FcεRl
High bind
monomeric IgE
Mast cells,
basophils,
eosinophils
Avtivation,
degranulation of
mast cell,
basophils
CLASSES (ISOTYPES) OF IMMUNOGLOBULINS

Classes based on constant region of heavy chains
 Immunoglobulin
A (IgA)
 Immunoglobulin D (IgD)
 Immunoglobulin E (IgE)
 Immunoglobulin G (IgG)
 Immunoglobulin M (IgM)

Differentiation of heavy chains
 Length
of C region, location of disulfide bonds, hinge region,
distribution of carbohydrate

Classes have different effector functions
CLASSES (ISOTYPES) OF IMMUNOGLOBULINS
Immunoglobulin G

Structure
 Monomer
(7S)
IgG1, IgG2 and IgG4
IgG3
IgG

It is the major Ig in normal serum, accounting for 75% of the total Ig pool. It
is a monomeric unit (2 heavy chain &2 light chain), MW 160,000. It can
bind 2 Ag molecules. 4 subclasses are known IgG1,2,3,4. Its biological
activities include:

Its half life time is 23 days and is the longest of all Igs.

It is the major Ab in the secondary immune response.




It is the only Ab that can cross placenta (IgG2 does not cross well) and
provide immunity to the newborn during the first months after birth. Transfer
is mediated by a receptor on placental cells for FC region of IgG .
It diffuse into the extravascular neutralizing bacterial toxins (antitoxin).
It enhance phagocytosis (opsonization) by coating bacteria and attaching
by its FC portion to FC receptor on phagocytic cells.
It can fix and activate complement (by IgG1 and IgG3)
Immunoglobulin A
Secretary Ab
First line defense
for microbes
IgA : a doublet guards the entrance to the body. 170,000 MW in serum
and 400,000 MW in external secretions, 15% of Ig in serum, found in
the blood as a monomer, and in tears, saliva, colustrum, nasal, vaginal,
prostatic and bronchial secretions as a dimer.
• Blocks attachment of microbes to mucous membranes
• It concentrates in body fluids such as tears, saliva, and secretions of
the respiratory and gastrointestinal tracts.
IgA

Structure
 Serum
- monomer
 Secretions (sIgA)
 Dimer (11S), sIgA molecule consists of two
H2L2 units plus one molecule each of J chain
and secretory component (SC or SP)
Secretory Piece
J Chain
IgA

Properties
 2nd
highest serum Ig
 Major
secretory
Ig
(saliva, tears, respiratory,
intestinal, and genital
tract secretions.)
 Does not fix complement
unless aggregated
 Binds to Fc receptors on
some cells
Immunoglobulin M
Macroglobulin
primary immune response
Bacteriolytic
•IgM usually combines in star-shaped clusters. pentamer,
•It tends to remain in the bloodstream, 10% of blood Ig,
• found on the surface of B lymphocytes.
•Activates the complement system.
IgM

Structure
 Pentamer
(19S)
composed
5
H2L2 units plus one
molecule of
J chain
 Extra domain (CH4)
 J chain
J Chain
C4
Fixation of C1 by IgG and IgM
Abs
No activation
Activation
IgM


Structure
Properties
3rd highest serum Ig
 First Ig made by fetus and B
cells
 Produced early in the
primary response
 The most efficient Ig
 Fixes complement

Agglutinating Ig
Binds to Fc receptors
B cell surface Ig
Tail Piece
Immunoglobulin D
Membrane bound antibody
Found on B-cell membrane
Memory function

180,000 MW, activity is not well known. 0.2% of plasma Ig. 13%
carbohydrate content. Also found on the surface of B lymphocytes,
where it somehow regulates the cell's activation
IgD

Structure
 Monomer
 Tail
piece
Tail
Piece
IgD


Structure
Properties
 4th
highest serum Ig
 Expressed in B cell surface Ig
 Does not bind complement

FUNCTIONS:
B
cell activation .
 Acts as receptor for Ag binding.
Immunoglobulin E
Activate mast cells
Release vasoactive amines
Respond to allergens
Mediate hypersensitivity
reactions




90 000 MW, largest immunoglobulin, present in extremely low levels in a healthy
individual.
IgE levels rise in response to parasitic infections and in allergic reactions.
Bind and activate mast cells. Mast cells cause acute inflammatory response (e.g.
swelling, redness, pain and itchiness).
Hay fever is a condition caused by too much IgE activity.
IgE

Structure
 Monomer
 Extra
domain (CH4)
C4
IgE


Structure
Properties
 Least
common serum Ig
 Allergic reactions
 Parasitic infections
 Does not fix complement

FUNCTIONS:
Responsible for immediate hypersensitivity or allergic reactions.
o Binds to Fc receptors on basophils and mast cells.
o Release of substances like histamine , bradykinin and other vasoactive
‘mediators’.
o
Properties of immunoglobulins:
IgG
IgA
IgM
IgD
IgE
1. Serum conc. (%)
85
5-15
5-10
<1
<1
2. Mol. Wt.
160,000
170,000 &
385,000
960,000
184,000
188,105
3.Sed. coeff.
7S
7S
19S
7S
8S
4.Heavy chain
class
Gamma
Alpha
Mu
Delta
Epsilon
5.Light chain
K&L
K&L
K&L
K&L
K&L
6. Valency
2
2 or multiple
of 2
5 (10)
2
2
Dimer or
Trimer
Pentamer
Monomer
Monomer
7.No of basic 4Monomer
polypeptide chains
IgG
IgA
IgM
IgD
IgE
8.Placental transport
+
_
_
_
_
9.Present in milk
+
+
_
_
_
10.Selectie secretion by
seromucus gland
_
+
_
_
_
11. Intravascular distribution(%)
45
42
80
75
50
12.Carbohydrate (%)
3
11
10
13
12
8.Subclasses
IgG1-4
IgA1-2
_
_
_
IgG
IgM
IgA
IgD
IgE
A.Classical
++
_
+++
_
_
B.Alternati
ve
_
+
_
_
_
15.Half life
(days)
23
6
5
2-3
2-3
16.Princip-al
site of
action
Serum
Secretion
Serum
Receptor for
B cells
Mast cells
17.characteristic
properties
precipitins,
antitoxins,
compleme-nt
fixation, late
Ab
Serum and
secretory
Abs
Agglutinin,
opsonin ,
lysin , early
Ab
Not known
(B-cell
activation)
Reaginic Ab
(anaphylaxis)
14.Complement
fixation
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