Structure of immunoglobulins

Presented by:-
STEFFI THOMAS
Assistant Professor
School of Pharmacy
LNCTU, Bhopal

 Immunoglobulin is a glycoprotein that is made in
response to an antigen and can recognize and
bind to the antigen that caused its production.
 Antibodies are the globular proteins belonging to
Immunoglobulin (Ig) family
 Are gamma globulins
 Synthesized by plasma cells
 Constitute 25-30% of total serum proteins
 Antibodies are present in serum, tissue fluids and
mucosal surfaces.
 All antibodies are immunoglobulins, but all
immunoglobulins may not be antibodies

 Based on structure and antigenic nature of H
chain the immunoglobulins are classified into
5 classes:-
1. Ig G- (gamma)
2. Ig A- (alpha)
3. Ig M- (mu)
4. Ig D- (delta)
5. Ig E - (epsilon)

 Composed of 4 polypeptide chains.
 Consists of 2 identical light chain (of 22000 Da) and 2
identical heavy chains (55000 Da)
 Linked by disulphide bonds and by non-covalent
interaction
 Light chains similar in all immunoglobulins
 Light chains occur in 2 varieties (2 types of constant
region sequences): kappa(ĸ) and lambda(λ)
 In a particular antibody either 2 lambda or 2 kappa
chains are present but not 1 lambda and 1 kappa
 Light and Heavy chains are subdivided into variable
and constant region.
 Each heavy and light chain contains amino terminal in
variable region and carboxy terminal in constant
region

 L-chain is composed of about 220 amino acids,
around 100-110 amino acids are located at N-
terminal (amino terminal) and the amino acid
sequences varies among antibodies. This region of
L-chain is known as variable region.
 H-chain consists of 110 amino acids located at N-
terminal. This region is known as Variable region.
 Heavy chains are structurally and antigenically
distinct for each class
 Each immunoglobulin peptide chain has intra
chain disulphide bonds- form loops
 Light chain contains a single Variable domain (VL)
and a single Constant domain (CL).
 Heavy chain contains one variable domain (VH)
and 3 constant domains (CH1, CH2, CH3)
 Hinge region is the segment in heavy chain -
between CH1, CH2

 Fab region-stands for “Fragment antigen
binding” where antigen binding takes place.
-Antigen binding is accomplished by amino-
terminal region and effector functions by carboxyl
terminal (C-terminal) region of antibody.
-In an antibody molecule 2 Fab regions are found
and they bind the antigens.
-VL domain (L-chain) and VH domain (H-chain)
form the antigen binding site.

 Fc region-it stands “Fragment crystallizable”
-It is so called because the well conserved amino acid
sequence allows this fragment to crystallize.
-It allows interaction of immune complex with other
phagocytic cells and complement.
-This region ensures that each antibody generates an
appropriate immune response for a given antigen.
-This region also binds to several cell receptors such
as Fc receptor.
-By doing this it mediates different physiological
effects such as opsonization (modification of antigens
by opsonin (which is a protein of innate and adaptive
immune system) to make them more accessible to
phagocytic cells and other immune cells), cell lysis
and degranulation of mast cells.

 Papain enzyme- Peptide bonds in the hinge region
are broken
-Produces 3 fragments 2 identical fragments called
Fab fragments – antigen binding activity.
-Other fragment called Fc fragment (Fraction
crystallizable)
 Pepsin digestion- Produce a single fragment
composed of two Fab like subunits F(ab) 2 binds
antigen
-Fc fragment is not recovered- digested to small
numerous peptide

 Most abundant class in serum
 Constitutes 80% total immunoglobulin
 Present in blood, plasma and tissue fluids
 Contains less carbohydrate than other
immunoglobulins
 It has a half life of 23 days: the longest of all of
the immunoglobulin isotypes
 Crosses placenta and provide natural immunity to
foetus and neonate at birth
 Acts against bacteria and viruses by opsonizing
 Neutralize toxin
 Activate complement by classical pathway
 Catabolism of IgG is unique in that it varies with
its serum concentration

 Constitutes 10-15 % of total immunoglobulins
 Present in milk, saliva, tears, mucous of
respiratory tract, digestive tract and genitourinary
tract.
 In serum exist as monomer
 In external secretions exist as dimer called
secretory Immunoglobulin.
 Has ‘J’ chain and secretory piece.
 Half life: 6-8 days

 Provides local immunity.
 Secretory Ig A binds to surface antigens of
microorganism and prevent its attachment and
invasion of the mucosal surfaces of respiratory and
digestive tract- immune elimination.
 Secretory IgA provides important line of defense
against Salmonella, Vibrio cholerae, N.
gonorrhoeae, influenza virus and poliovirus.
 Secretory IgA present in breast milk protects
newborn during first months of life.
 Promotes phagocytosis and intracellular killing of
microorganisms

 Accounts for 5-10% of total serum proteins
 Polymer of five monomeric units (pentamer)
 Held together by disulfide bonds and ‘J’ chain
 Mol. Wt. of 900,00010,00,000 (millionaire
molecule)
 Half life: 5 days
 Most of IgM (80%) present intravascularly
 Present in low concentration in intercellular tissue
fluids
 Cannot cross placenta
 Presence of IgM antibody in serum of newborn
indicate congenital infection.
 Earliest immunoglobulin to be synthesized by
foetus (20 weeks)

 First immunoglobulin to be produced in primary
response to antigen
 Relatively short-lived hence it’s demonstration in
the serum indicates recent infection
 Monomeric IgM appears on the surface of
unstimulated B lymphocytes and act as receptors
for antigens
 Functions
- It agglutinates bacteria
- Activates complement by classical pathway
- Causes opsonization and immune hemolysis
- Believed to be responsible for protection against
blood invasion by microorganisms

 Structure is similar to Ig G
 Has 4 constant region domains.
 Mol. Wt. 1,90,000
 Heat labile (inactivated at 56ºC in 1 hour)
 Normal serum concentration 0.3 ug/ml
 Mostly present extra cellularly
 Does not cross placenta

 Produced in the lining of respiratory and
intestinal tract
 Known as reagin antibody
 Does not activate complement nor agglutinate
antigens
 Binds to the Fc receptors on the membranes of
blood basophils and tissue mast cells
 Mediates immediate hypersensitivity reaction
and P.K. reaction
 Responsible for symptoms of anaphylactic
shock, hay fever and asthma.
 Play a role in immunity against helminthic
parasites

 Structure is similar to IgG
 Serum concentration 30 micrograms per ml
 Constitutes 0.2% of total immunoglobulins
 IgD together with IgM is major membrane
bound immunoglobulin on unstimulated B
lymphocytes-acts as recognition receptors for
antigens

 IgG: Protects the body fluids
 IgA: Protects the body surfaces
 IgM: Protects the blood stream
 IgE: Mediates type I hypersensitivity
 IgD: Role not known

Structure of immunoglobulins

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Structure of immunoglobulins