Download Imunoglobulins

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)
• 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.
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: Ig G is the most important antibodies in fighting bacteria and virus
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 endogenous 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
as 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, secretions of GIT
It protects against infection of mucus membranes
that lining mouth, GIT and air ways.
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 i Times New Roman s the first Ig be made by a virgin B cells
when it is stimulated by antigen. Then other classes are
synthesized.
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) It plays an important role in allergic response to allergens (antigen
causes allergy or hypersensitivity such as dust or pollens). Ig E, in
the presence of alleregen, 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.
Ig G
Ig A
IgM
Ig D
Ig E
160
160
970
184
184
% of total Ig
70-75
10-15
5-10
Less
than 1
trace
Cross placenta
yes
NO
NO
NO
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
Mass (KD)