Antibodies
are substances which are formed in the serum and tissue fluids in response to
an antigen and react with that antigen specifically and in some observable
manner.
A. Properties of antibodies :
Chemical nature of antibodies is
globulin and they are named as immunoglobulins . immunoglobulins are mainly
synthesized by plasma cells.
B. Structure of immunoglobulin :-
1. An
antibody molecule consists of two identical heavy and two identical chains.
2. The
heavy (H) chains are shorter. Both types of chains are polypeptide in nature.
3. The
two heavy chains are held together by disulphide (S-S) bonds.
4. Each
light chain is also attached to heavy chain by disulphide bond.
5. The
H chains are structurally and antigenically distinct in different classes
namely IgG, IgM,IgA,IgD, and igE are designated depending on presence of heavy
chain, gamma (¥) , mu(µ), alpha (α), delta (Ꝺ) and epsilon (€) respectively.
6. The L chains are
similar in all classes of
immunoglobulins.
7. They
are present in two forms kappa (K) and lambda (L).
8. Each
immunoglobulin has either two kappa or two lambda light chains but both
(K&L) are never found together in a molecule.
9. Both
L and H chains consist of two portions each, a variable (V) region and a
constant (C) region.
10. Antigen
combining site is at its amino terminus which consists of both H and L chains.
Classification :
Immunoglobulin are classified into 5 types.
1. Immunoglobulin
G (IgG)
2. Immunoglobulin
A (IgA)
3. Immunoglobulin
M (IgM)
4. Immunoglobulin
D (IgD)
5. Immunoglobulin
E (IgE)
1.
Immunoglobulin G (IgG)
·
IgG is the major serum immunoglobulin .
the normal serum concentration is about
8-16 mg/dl.
·
Molecular weight is 150,000 (7S).
·
It is the only immunoglobulin that is
transported through placenta and provides natural passive immunity to newborn.
·
It is distributed equally between the
intravascular and extravascular compartments.
·
IgG appears late but persists for longer
period. It appears after the initial immune response which is IgM in nature.
·
It participates in precipitation,
complement fixation and neutralization of toxin and viruses.
·
It is protective against those
microorganisms which are active in the blood and tissues.
2.
Immunoglobulin A (IgA)
·
IgA is the second major serum
immunoglobulin (about 10-13% of serum immunoglobulins). The normal serum
concentration is 0.6- 4.2 mg/ml.
·
IgA occurs in two forms, serum IgA and
secretory IgA.
·
Serum IgA is a monomeric 7S molecule ,
while IgA found on mucosal surfaces and in secretions ( secretory IgA, MW 400,000)
is a dimer formed by two monomer units joined
together by a glycoprotein named the J chain (J for joining).
·
Secretory IgA contains another
polypeptide called the secretory piece or secretory component.
·
IgA is the principal immunoglobulin
present in secretions such as milk, saliva, tears, sweat, nasal fluids,
colostrum and in secretions of respiratory intestinal and genital systems. It
protects the mucous membranes against microorganisms.
·
IgA is mainly synthesized locally by plasma cells and little
is derived from serum.
3.
Immunoglobulin M (IgM)
·
IgM is a pentamer consisting of 5
immunoglobulin subunits and one molecule of J chain, which joins the Fc region of the basic
subunits.
·
It constitutes about 5-8 percent of
total serum immunoglobulins. The normal level in serum is 0.5 – 2 mg/ml.
·
It is heavy molecule (19S) with a
molecular weight 900,000 to 1,000,000 hence also called the ‘ millionaire
molecule’.
·
IgM is mainly distributed
intravascularly (80%).
·
It is the earliest synthesized
immunoglobulin by foetus.
·
It appears early in response to
infection before IgG. IgM antibodies are
short lived,and disappear earlier than IgG. Hence , its presence in serum
indicates recent infection.
·
It cannot cross the placenta , presence
of IgM antibody in serum indicates congenital infection.
·
It is very effective antibody in
agglutination and complement fixation. It is more efficient than IgG in these
reactions.
·
IgM provides protection against blood
invasion by microorganisms.
4.
immunoglobulin D (IgD)
·
IgD resembles IgG structurally .
·
IgD is present in a concentration of 3
mg per 100 ml in serum. It is mostly intravascular in distribution.
·
IgD (175kD) constitutes less than 1%
(40mg/ml) of the antibody in human serum.
·
IgD is an antibody whose function
remains unknown, even though it is one of the main receptors on mature B cells.
·
As B cells mature, IgD is replaced by
other antibodies.
·
igD may be a regulator of immune
responses.
·
The hinge region of IgD consists of 64
amino acid residues, longer than any other antibody class.
5.
Immunoglobulin E (IgE):-
·
Human IgE (190 kD) makes up less than
0.003% of the antibody in serum.
·
IgE binds through its Fc part to mast
cells or basophils.
·
On later exposure to the same antigen,
that antigen may be bound on mast cells and basophils with membrane – bound on
mast cells and basophils with membrane – bound IgE and trigger allergic
reactions.
·
As it mediates the immediate hyper
sensitivity .
·
Reaction it is called regain antibody
·
IgE protects against parasites by
releasing mediators that attract eosinophil.
·
Like the m chain, the 5 chain contains
four C-region domains.
·
igE is made up of about 13% of
carbohydrate.
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