Immunoglobulin classes

IMMUNOGLOBULIN CLASSES
R. RENUKA, M.SC.,M.PHIL.,PH.D.
ASSOCIATE PROFESSOR OF BIOCHEMISTRY,
V.V.VANNIAPERUMAL COLLEGE FOR WOMEN,
VIRUDHUNAGAR, TAMILNADU, INDIA.

HUMAN Ig CLASSES
Igs can be differentiated into 5 different classes based on the differences
in amino acid sequences in constant region of the heavy chain.

1. Ig G
 IgG, a monomer, is the predominant Ig class
present in human serum which constitute
approximately 75% of total serum Ig.
 Produced as part of the secondary immune
response to an antigen.
 IgG is the only class of Ig that can cross the
placenta in humans, and it is largely
responsible for protection of the newborn
during the first months of life.
 Because of its relative abundance and
excellent specificity toward antigens, IgG is the
principle antibody used in immunological
research and clinical diagnostics.

PROPERTIES OF Ig G
 H – chain type is gamma.
 Molecular weight is 1,50,000.
 Sedimentation coefficient is 7S.
 Present in extra and intravascular spaces.
 Longest life span of 23 days.
 Contains less carbohydrates than other Igs (3%).
 It has 4 sub-classes; Ig G1, Ig G2, Ig G3 and Ig G4. All different sub
classes vary in number of disulphide bridges, length of hinge region
and biological functions.

Ig G sub classes in human
Note the arrangement and numbers of disulphide bonds

FUNCTIONS OF Ig G
 Generally its immune response appears late and persists for longer time.
 Ig G3 is the only Igs which can cross placenta and provide immunity to foetus and
new born.
 Plays major role in neutralization of toxins and viruses as it can easily diffuse into
extravascular space.
 Ig G1 and Ig G3 bind to Fc receptors on phagocytes with high affinity (opsonins) and
enhances the phagocytosis of antigen bound to it.
 Fc region can bind to complement factors, activate classical pathway of
complement system.
 It is bactericidal, viricidal and mediating precipitation reactions.
 It is also associated with type II and type III hypersensitivity reactions.

2. Ig A
 Second most abundant Ig.
 2 sub classes: Ig A1 and Ig A2. They differ in the
molecular mass of the heavy chains and in their
concentration in serum.
 IgA exists in two forms: IgA in serum is mainly
monomeric, but in secretions, such as saliva,
tears, colostrum, mucus, sweat, and gastric
fluid IgA is found as a dimer connected by a
joining peptide. Most IgA is present in secreted
form.
 The principal function of secretory IgA may be
not to destroy antigens but to prevent passage
of foreign substances into the circulatory
system.

DIMERIC FORM OF Ig A
 IgA found in secretions is a dimer having a J chain. Secretory
IgA also contains a protein called secretory piece or T-
piece, this is made in epithelial cells and added to the IgA as
it passes into secretions helping the IgA to move across
mucosa without degradation in secretions

Properties of Ig A
 Molecular weight: 3,20,000 (secretory)
 H-chain type (MW): alpha (55,000)
 Serum concentration: 1 to 4 mg/mL
 Percent of total immunoglobulin: 15%
 Glycosylation (by weight): 10%
 Half life period: 6 days
 Distribution: intravascular and secretions
 Function: protect mucus membranes

Functions of Ig A
•IgA can’t opsonize antigen, it does not activate complement cascade, but
it can agglutinate antigen and neutralize viruses with lysozymes.
•Serum Ig A interacts with Fc receptors expressed on immune effector cells,
to initiate various functions like ADCC, degranulation of immune cells, etc.

3. Ig M
 It normally exists as a
pentamer in serum but can
also occur as a monomer on
B lymphocytes as B cell
receptors.
 It has an extra domain on
the mu chain (CH4) and
another protein covalently
bound via S-S bond called
J-chain. This chain helps it to
polymerize to the pentamer
form.

PROPERTIES OF Ig M
 It is the 3rd most abundant Ig in serum. Present only in intravascular
compartments and not in body fluids and secretions.
 Heavy chain type is Mu.
 Molecular weight is 9,00,000.
 Serum concentration is 0.5 to 2mg/ml and constitute 10% of total Igs.
 Sedimentation coefficient is 19S.
 Half life period is only 5 days.
 It has 10 antigen binding site i.e. valency is 5.
 The 5 subunits arrange in a pin wheel array with the Fc portion in the centre.
 Glycosylation (by weight) is12%.

FUNCTIONS OF Ig M
 First Ig to be produced in primary response to antigen (primary
response ab)
 Relatively short lived hence its presence in the serum indicates recent
infection.
 It can not cross placenta. So, presence of Ig M ab in serum of new born
indicates congenital infections (intrauterine) such as congenital
syphilis, rubella, toxoplasmosis etc.
 Present on B cell surface and serves as B cell receptor for Ag binding.
 Give protection against intravascular organisms.
 IgM is the only antibody made to certain carbohydrate antigens, such as
the ABO blood group antigens on human erythrocytes.
 It also shows properties such as opsonization, complement fixation,
agglutination, cytolysis, etc.

5. Ig E
 It is found in low concentration. It is present in serum
of patients with certain types of allergies. These
antibodies are also known as allergens.
 Molecular weight is 190000 to 200000 with a short
half life of 2.5 to 3 days.
 It is cytophilic antibody and is attached to mast cells
and basophils through its Fc portion. It mediates the
release of inflammatory molecules.
 It is also known as reaginic antibody.
 IgE antibody is mainly formed by plasma cells. found
mostly in secretory surface and formation is
influenced by T- cell.

Properties of Ig E
 Structure similar to Ig G.
 Has 4 constant region domains.
 Molecular weight: 1,90,000
 H-chain type (MW): epsilon (73,000)
 Serum concentration: 10 to 400 ng/mL
 Percent of total immunoglobulin: 0.002%
 Half life: 2 days
 Glycosylation (by weight): 12%
 Heat labile
 Distribution: basophils and mast cells in saliva and nasal secretions
 Function: protect against parasites

Overall Functions
 Based on antigen recognition and binding:-
 1. mAb as B-cell receptor
 2.sAb as antigen neutralizing agent
 Based on effector response:-
 1. Complement activation
 2.Opsonization
 3. ADCC
 Based on Ig class:-
 1. Neonatal immunty
 2.Mucosal immunity
 3. IgE mediated hypersensitivity reaction.

Properties of Immunoglobulins:

Isotypes, Allotypes and Idiotypes of
Antibodies
 Antibodies are glycoproteins in nature and therefore they are good
antigens.
 They can induce immune response even in a genetically closely related
host.
 This immune response is generated due to the antigenic determinants
(epitopes) present on constant as well as variable regions of heavy and
light chains.
 Depending on the nature of the response generated by these antigenic
determinants, antibodies are classified into three types: isotypes, allotypes
and idiotypes.

Isotypes
 All the 5 Ig classes and sub classes are called isotypes.
 Ig molecules of each class and sub class in a given species carry certain amino acid sequences on
the constant regions of both heavy and light chains, which are called isotypic determinants.
 These are heavy chain class and sub class and light chain type and sub type specific and also
species specific.
 That means, the isotypic determinants on Ig G1 will differ in mouse and human and isotypic
determinants on human Ig G1 will differ from those on human Ig G2.

Allotypes
 Allotype determinants are present both on CH and CL regions, which differ from one individual
to another in a given species (exception: members of the same inbred strains of mice and
monozygotic twins) .
 The allotype determinants on 𝛾 heavy chain are referred to as Gm markers. They are present
in all sub classes of 𝛾 heavy chain.
 Allotypic differences are very little, differing only in 1-4 amino acids.

Idiotypes
 Idiotypic determinants are present in the VH
and VL regions, either in the frame work
region or in CDRs.
 The variable sequences which are small
stretches of polypeptides, are considered as
foreign by the host of the same species or
even by the host in which antibodies are
produces, and the host can mount immune
response to them. Such determinants are
called as idiotopes.

Immunoglobulin Super Family
 Many of the membrane proteins which participate mainly in immune response have similar folded
domain structures like that of Igs.
 The domain consists of about 110 amino acids and contain an intrachain disulphide bond forming a
loop of 50- 70 amino acids, similar to antibody domains.
 Such proteins with regions homologous to immunoglobulin domains are said to belong to the Ig
super family.
 Members of Ig SF include cell surface ag receptors, coreceptors and costimulatory molecules of the
immune system, molecules involved in ag presentation to lymphocytes, cell adhesion molecules
and certain cytokine receptors.

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