ANTIBODY
Babitha Elias
DEFINITION
Antibodies are substances which are formed
in the serum or tissue fluids in response to
an antigen.
 Antibodies react with antigen specifically
and in an observable manner.
Immunoglobulin – proteins of animal origin,
endowed with known Ab activity & for
certain other proteins related to them by
chemical structure.
Properties of antibodies
 Chemical nature of antibodies is globulin –
immunoglobulin
 Constitute about 20 – 25% total serum
proteins.
 Based on sedimentation studies – 7S (M.W
– 1,50,000- 1,80,000 KD). Heavier
antibodies – 19S globulins (M.W –
900,000)
 Kabat showed, on electrophoretic mobility,
the antibodies belong to gammaglobulins
Kabat, 1939
Structure of immunoglobulins
 Porter, Edelman and Nisonoff
 Antibody molecule consists of two identical
heavy chains (H) and two identical light
chains (L).
 Heavy chains are longer & light chains are
shorter.
 Both chains are polypeptide in nature.
 2 heavy chains are held together by
disulphide bonds.
 Each light chain is attached to a heavy
chain by disulphide bond.
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The H chains are structurally and
antigenically distinct in different classes of
immunoglobulins.
5 different classes of immunoglobulins
depending on the presence of heavy
chain.
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Ig
Ig
Ig
Ig
Ig
G–γ
M-μ
A-α
E-ε
D-δ
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The L chains are similar in all classes of
immunoglobulins.
Present in two forms – kappa (K) and lambda (L).
Effect of enzymes on Immunoglobulins
Papain Digestion
 Papain can digest immunoglobulin molecule into 3
fragments – one Fc (Fragment crystallisable) and
2 identical Fab (fragment antigen binding)
 The 2 Fab fragments possess the antigen binding
sites but the Fc fragment lacks the ability to bind
antigen.
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Each Fab fragment consists of a light (L)
chain & a part of H chain
Portion of H chain in Fab fragment – Fd
region
Fc fragment consists of both the H chains.
It determines the biological properties of
the Ig – complement fixation, placental
transfer etc.
Pepsin digestion
 Pepsin cleaves immunoglobulin into 1 Fc
portion & 2 Fab fragments held together in
position.
 The Fab is bivalent & still it can precipitate
with antigen – F(ab’)2
 Pepsin degrades the Fc portion into
smaller fragments.
Pepsin digestion
H chains & L chains
 Consists of two portions – a variable
region (V) & a Constant region (C).
 In the L chain, the 2 regions are of equal
length, while in the H chain, the V region
constitute only a fifth of the chain.
 Variable regions are in the amino terminus
(NH2) and Constant region at
carboxyterminus (COOH)
IMMUNOGLOBULIN CLASSES
IMMUNOGLOBULIN G (Ig G)
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Major serum immunoglobulin
– 80% of total amount.
The normal serum
concentration – 8-16 mg/ml.
Molecular weight – 150,000
(7S)
Half life – 23 days
It can cross the placenta –
Transplacental
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It is distributed equally between the
intravascular & extravascular
compartments.
Ig G appears late but persists long. It
appears after the initial immune response.
It participates in precipitation,
complement fixation, & neutralization of
toxin & viruses.
Ig G binds to microorganisms & enhances
phagocytosis.
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Catabolism of Ig G –
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When Ig G level is raised, the synthesis of Ig G against
that particular Ag is catabolised rapidly & result in
deficiency of particular Ab. Eg: myeloma & kala-azar.
In hypogammaglobulinaemia, Ig G antibody given for
therapeutic purpose will be catabolised slowly.
Passively administered Ig G suppresses the
homologous antibody synthesis by feed back
mechanism.
4 sub classes of Ig G – Ig G1, Ig G2, Ig G3, Ig G4
It is protective against microorganisms which are
active in the blood and tissues.
IMMUNOGLOBULIN A (Ig A)
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Second major serum immunoglobulin – 1013%.
Normal serum concentration – 0.6-4.2 mg/ml.
Half life – 6 – 8 days.
Ig A occurs in two forms – Serum Ig A &
Secretory Ig A.
Serum Ig A is a monomeric 7S molecule.(MW:
160,000), While Ig A in the mucosal surfaces &
secretions (Secretory Ig A, MW: 400,000) is a
dimer. It is formed by two monomer units joined
together by a glycoprotein – J chain
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J chain – joins the 2 monomers at their
carboxy terminals. Secretory Ig A & J chain
are produced by the plasma cells.
Secretory Ig A contains a secretory piece, S
piece is synthesised by mucosal & glandular
epithelial cells. The S piece protects Ig A from
denaturation by bacterial proteases in sites such
as intestinal mucosa.
Ig A is present in secretions such as milk, saliva,
tears, sweat, nasal fluids etc. It protects the
mucus membranes against microorganisms..
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Ig A cannot fix complement, but can
activate alternative complement pathway.
Ig A is synthesised locally by plasma cells.
2 subclasses of Ig A – Ig A1 & Ig A2
IMMUNOGLOBULIN M (Ig M)
 Ig M is a pentamer consisting of 5
monomers joined by a J chain.
 It constitutes about 5-8% of total serum
concentration.
 Normal level – 0.5-2 mg/ml
 Half life – 5 days.
 Molecular weight – 19S (900,000-1,000,000)
 ‘Millionaire molecule’
 Distributed intravascularly.
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It is the earliest synthesised Ig by foetus in
about 20 weeks of age.
It appears early in the infection before Ig G.
Ig M Abs are short lived – presence
indicates recent infection.
It cannot cross placenta – presence in new
born indicates congenital infection.
It participates in agglutination, complement
fixation, opsonisation & immune hemolysis
Ig M provides protection against blood
invasion by microorganisms.
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Ig M monomers appear on the surface of
unstimulated B lymphocytes & act as
receptors for antigens.
Two subclasses – Ig M1 & Ig M2.
IMMUNOGLOBULIN E (IgE)
 Produced in the lining of respiratory &
intestinal tracts.
 Also known as “Reagin”
 MW- 190,000 (8S)
 Half life 2-3 days
 Resembles Ig G structurally
 Heat labile (Inactivated at 56 o C for 1 hr)
 It has affinity for surface of tissue cells –
Mast cells .(Homocytotropism)
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Ig E mediates Type-I hypersensitivity
Responsible for asthma, hay fever, eczema,
Prausnitz-Kustner (P-K) reaction.
Cannot cross the placental barrier.
Plays role in defense against parasitic
infection.
IMMUNOGLOBULIN D (Ig D)
 It resembles Ig G structurally.
 Serum concentration – 0.03 mg/ml
 Intravascular in distribution.
 MW – 180,000 (7S)
 Half life – 3days
 It also act as receptors for antigen, like
monomeric IgM .
 2 subclasses –IgD1, IgD 2
~0.2%
~80%
~0.002%
~5-10%
~10-15%
(High daily
Production)
Fig. 4-
Role of different Ig classes
IgG – Protects the body fluid
IgA - Protects the body surface
IgM – Protects the blood stream
IgE - Mediates reaginic hypersensitivity
IgD- Recognition molecule on the surface of
B lymphocytes
ABNORMAL IMMUNOGLOBULINS
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Apart from antibodies, other structurally
similar proteins may be found in the
serum in the following conditions.
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Multiple myeloma
Heavy chain disease
Cryoglobulinaemia
Multiple myeloma
 It is a plasma cell dyscrasia in which
unchecked proliferation of one clone of
plasma cells resulting in the excessive
production of particular Ig.
 Multiple myeloma involve plasma cells
synthesizing any of the 5 classes of Ig.
 Multiple myeloma involving IgM producing
plasma cells – “Waldenstrom’s
Macroglobulinaemia”
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Excessive production of light chains –
Bence Jones proteins.
It can be identified in urine – proteins get
coagulated at 50oC but redissolve at 70oC
Heavy chain disease
 Abnormal heavy chains are produced in
excess. This is due to lymphoid neoplasia.
Cryoglobulinaemia
 It is a condition in which there is
formation of precipitate on cooling the
serum, which redissolves on warming.
 Found in macroglobulinaemia,
autoimmune conditions like SLE.
 Most cryoglobulins consists of either Ig G
or Ig M.
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