1. Inter-chain disulfide bonds

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Transcript 1. Inter-chain disulfide bonds

Antibodies (Immunglobulins (Igs)
A- Definition:
• Immunoglobulins are a group of proteins (gamma globulins) produced
by the body from B lymphocytes and plasma cells in response to
presence of foreign bodies (antigens).
• There are 5 basic types: G, M, A, E and D.
• All of immunoglobulins have a similar basic structure.
B- Basic structure of immunoglobulins:
• The basic unit of all Igs molecules consists of 4 polypeptide chains
linked by disulfide bonds:
a) 2 polypeptide chains of low molecular weight called light chains (L)
b) 2 polypeptide chains of high molecular weight called heavy chains (H)
• Disulfide bonds in Ig:
• 1. Inter-chain disulfide bonds - heavy(H) and light (L) chains
are linked together by disulfide bond. In addition, the two heavy
chains are held together by inter-chain disulfide bonds and by
non-covalent interactions.
• 2. Intra-chain disulfide bonds - Within each of the polypeptide
chains (within H and L chains) there are also intra-chain disulfide
bonds.
•
Enzyme called “papain” cleaves Ig into two fragments: Fab
fragment
and Fc fragment. Fab fragment is the part of
antibody molecule which combine with antigens.
Basic structure of Ig
Immunoglobulin classes
The Igs can be divided into five different classes, based on differences in
the amino acid sequences in the heavy chains.
Ig G
a) IgG is the major Ig in serum:70-75% of serum Ig is IgG
b) IgG is composed of one basic unit (monomer), i.e of low Mol. weight
c) Placental transfer: IgG is the only class of Ig that crosses the placenta.
In addition, Ig G is also transferred from mother to baby in breast milk.
Transplacental and breast milk-derived Ig G offer the baby a passive
immunologic protection until the endogeenous Ig G is produced. The
half life of Ig G is about 30 days and with prolonged breast feeding the
infant can get additional protection.
IgA
1. Structure
- IgA is a monomer in serum but found in
secretions is a dimer as presented in the
Figure. In diameric IgA, the molecules are
joined by a small polypeptide chain called
J chain linked to Fc regions.
- When IgA is found in secretions:, it also has
another protein associated with it called the
secretory piece or T piece. The secretory piece
helps IgA to be transported across mucosa and also
protects it from degradation in the secretions.
2. Properties
a) IgA is the 2nd most common serum Ig (10-15%).
b) IgA is the major class of Ig in secretions - tears,
saliva, colostrum, mucus.
c) can’t pass placenta
IgM
1.Structure
IgM normally exists as a pentamer. So it has the highest molecular
weight. The five units are linked by small polypeptide chain called J
chain
2. Properties
a) IgM is the third most common serum Ig (5-10% of total Ig).
b) IgM is the first Ig be made by a virgin B cells when it is stimulated
by antigen.
c) It can’t cross placenta
IgE
1. Structure: IgE exists as a monomer.
2. Properties:
a) is the least common Ig in serum
b) It is largely responsible for immunity against parasites . Since
serum IgE levels rise in parasitic diseases, measuring IgE levels is
helpful in diagnosing parasitic infections.
c) Ig E, in the presence of antigen, binds to mast cells and basophils
causing release of histamine and other substances from mast cells.
These substances result in allergic manifestations.
Ig D:
IgD exists only as a monomer. IgD is found in low levels
in serum; its role in serum uncertain.
Mass (KD)
Ig G
Ig A
IgM
Ig D
Ig E
160
160
970
184
184
5-10
trace
% of total Ig 70-75 10-15
Cross placenta
yes
NO
NO
Less
than 1
NO
In breast milk
yes
yes
NO
NO
NO
In secretions
NO
yes
NO
NO
NO
Binding with
mast cells
NO
NO
NO
NO
yes
NO
Electrophoresis:
It is the movement of charged molecules in an electrical field
towards the oppositely charged electrode.
It is used for separation of proteins for dignosis of some diseases
such as immunity disease , genetic diseases ( such as Hb S and Hb C
diseases.
The rate of migration of charged molecules depends on:
a - amount of charge on the molecule
b- molecular weight (density) of the molecule.
Serum protein electophoresis:
In the standard method, serum samples are applied to support
medium ( agarose gel) on plate close to the cathode end.
Both ends of support medium is immersed in alkaline buffer (pH
8.6), so proteins in this pH will be negatively charged.
2) The support strip is connected to two
electrodes and a current is passed throughthe
strip to separate proteins.
3) As serum proteins are negatively charged,
they
will move toward anode (positive
electrode). Serum proteins move according to
their density and
amount of charge then
arranged into 5 bands: albumin moves faster to
the anode followed by α1 globulin, α2 globulin,
β globulin and γ globulins.
The separated proteins are made visible by staining.
4) Density of each band is directly proprtional to its serum
concentration, so albumin will show the most dense band ( serum
albumin is 3.5-5.5 gm%, while globulins 1.8-3.3 gm%).
 In case of decreased serum albumin (hypoalbuminuria), the
albumin band becomes less dense. This occurs ,for example, in
advanced liver disease as liver is the site of albumin synthesis.