Immunoglobulin: Introduction, Types, Function, Isotypes, Allotypes, Idiotyp

Immunoglobulin: Introduction, Types, Function, Isotypes, Allotypes, Idiotypes , Polyclonal and Monoclonal antibodies

Introduction of Immunoglobulin

Also called antibody

Immunoglobulin is a glycoprotein that is made in response to an antigen and can recognize and bind to the antigen that caused its production.

Protects us from microbial infection.

Are gamma globulins

Synthesized by plasma cells

Constitute 25-30 % of total serum proteins

Antibodies are present in serum, tissue fluids, and mucosal surfaces and on the surface of B-cells where they act as antigen receptors.

The basic structure of Antibody

Immunoglobulin is composed of 4 polypeptide chains.

2 identical light chains (25 kDa each) and 2 identical heavy chains (50-73 kDa each)

Linked by disulfide bonds

Light chains similar in all immunoglobulins

Light chains occur in 2 varieties:-kappa(k) and lambda( λ )

Kappa chains are more frequently found.

Heavy chains:- gamma, alpha, mu, delta and epsilon.

One Ig contains one type of light chain and one type of heavy chain..(each 2/2)

The variable and constant region

Light and Heavy chains are subdivided into variable and constant regions.

Each heavy and light chain contains amino-terminal in variable region carboxy-terminal in the constant region

The variable region extends from N-terminal and the sequence in this region is highly variable.

The constant region extends from the end of the variable region to C-terminal and the amino acid sequence is relatively constant.

Heavy chains are structurally and antigenically distinct for each class.

L and H chains are linked together by both inter and intrachain S-S bonds.

H and L chain domains:-

Each H and L chain is made up of several small but similar regions called domains.

L- chain:- two-domain (VL and CL)

H-chain:- 4 domains in IgA, IgD, IgG (VH, CH1, CH2, and CH3) while 5 domains in IgM, IgE ( VL, CH1, CH2, CH3, and CH4).

Immunoglobulin fold:-

Folded loop-like structure

Hinge region:-

H-chain of arms extends into the hinge region.

Rich in proline and cystine.

Disulfide bond.

Digestion with proteolytic enzymes

Papain cleavage occurs above the S-S bond of the hinge region.

Produces 3 fragments

2 identical fragments called Fab fragments –antigen-binding activity.

Another fragment called Fc fragment (Fraction crystallizable)

Pepsin digestion

Pepsin cleavage occurs below the S-S bond of the hinge region.

Produce a single fragment composed of two Fab like subunits F(ab)2 binds antigen

Fc fragment is not recovered- digested to small numerous peptides.

Classification of Immunoglobulin

Based on the structure and antigenic nature of the H chain the immunoglobulin is classified into 5 classes.
IgG (gamma)
IgA (alpha)
IgM (mu)
IgD (delta)
IgE (epsilon)

Immunoglobulin G (Ig G)

A most abundant class of Ig 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 fetus and neonate at birth
Acts against bacteria and viruses by opsonizing
Neutralize toxin
Activate complement by the classical pathway

Subclasses of IgG

Ig G1, Ig G2, Ig G3, Ig G4.

Immunoglobulin M (Ig M)

Accounts for 5-10% of total serum proteins
The polymer of five monomeric units (pentamer)
Held together by disulfide bonds and ‘J’ chain
Mol. Wt. of 900,000-10,00,000 (millionaire molecule)
Half-life: 5 days
Most of IgM (80%) present intravascularly
Present in low concentration in intercellular tissue fluids
Cannot cross the placenta
The presence of IgM antibody in serum of newborn indicate congenital infection.
Earliest immunoglobulin to be synthesized by the fetus (20 weeks)
First immunoglobulin to be produced in the primary response to antigen
Relatively short-lived hence its demonstration in the serum indicates recent infection
Monomeric IgM appears on the surface of unstimulated B lymphocytes and acts as receptors for antigens

Functions

It agglutinates bacteria
Activates complement by the classical pathway
Causes opsonization and immune hemolysis
Believed to be responsible for protection against blood invasion by microorganisms

Immunoglobulin A (Ig A)

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

Functions

Provides local immunity.
Secretory Ig A binds to surface antigens of microorganisms and prevents its attachment and invasion of the mucosal surfaces of the respiratory and digestive tract- immune elimination.
Secretory IgA provides an important line of defense against salmonella, Vibrio cholerae, N. gonorrhoeae, influenza virus, and poliovirus.
Secretory IgA present in breast milk protects newborns during the first months of life.
Activates complement by the alternative pathway
Promotes phagocytosis and intracellular killing of microorganisms

Immunoglobulin D (Ig D)

The structure is similar to IgG

Serum concentration 30 micrograms per ml

Constitutes 0.2% of total immunoglobulins

Half-life: 3 days

IgD together with IgM is major membrane-bound immunoglobulin on unstimulated B lymphocytes-acts as recognition receptors for antigens

Immunoglobulin E (Ig E)

The structure is similar to IgG

Has 4 constant region domains.

Mol. Wt. 1,90,000

Half-life: 2 days

Heat labile (inactivated at 560C in 1 hour)

Normal serum concentration 0.3 ug/ml

Mostly present extracellularly

Does not cross the placenta

Produced in the lining of the respiratory and intestinal tract

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

Play a role in immunity against helminthic parasites

Properties of immunoglobulins

Overall Functions of Antibodies

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 immunity

2. Mucosal immunity

3. IgE mediated hypersensitivity reaction.

Antigenic determinant of Antibodies

Antibodies are complex glycoproteins and they themselves can act as immunogens and induce antibody formation against them called anti-Ig antibodies.
The whole Ig molecule is not immunogenic to the host system which produces antibodies to them, rather small sites or regions on the Ig molecule acts as immunogen.
These sites or regions are called antigenic determinants.

Based on their location on Ig, they are classified into three types:-isotypes, allotypes, and idiotypes.
Isotypes:-formed by a unique sequence of amino acids located in the constant region of the H and L chain.
Hence different isotypes differ from each other in their constant regions.
Classes and subclasses of Igs are isotypes of one another.
Isotypes are present in all the members of the same species and they are the same in all.
Isotypes are different in different species. e.g. IgG of the mouse is different from the IgG of rabbit.
The simplest way of producing isotype antibodies is to inject antibodies from one species to another.

Allotypes

The antigenic determinant is present in the constant region of H and L chains and is encoded by polymorphic alleles, which are called allotypes.
Since some members of a species carry alleles not all, they are present in some members of species.
Allotypes differ in sequences of one to four amino acids from one another.

Idiotypes

Are located in the hypervariable region of the VH and VL domains and one member of a species acts antigenic determinant to another member of the same species.

Polyclonal and Monoclonal antibodies

An antigen usually has many epitopes and each epitope entering the body may stimulate a specific B-cell whose membrane receptor recognizes and binds to that epitope.
So, many B-cells, each with unique specificity to one particular epitope are stimulated.
Each stimulated B-cell produces antibodies specific to that one particular epitope.
Therefore, the Serum of such an immunized subject contains a mixture of antibodies –specific to various epitopes present on an antigen.
Such a mixture of antibodies is called polyclonal antibodies because it is derived from many B-cell clones contains Abs of different specificities.
However only one B-cell is specifically stimulated by one particular epitope and is then allowed to proliferate and produce antibodies, then these Abs have two inherent characters: -They all are derived from one single B-cell and its clone. They all have the same antigenic specificity and such antibodies are called monoclonal antibodies.
So monoclonal antibodies can be defined as the antibodies with identical antigenic specificity derived from a single B-cell clone.
Monoclonal antibodies are produced by hybridoma technology.

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