This document discusses immunoglobulins (antibodies) and their structure. It notes that antibodies are Y-shaped proteins produced by plasma cells to neutralize pathogens. Each antibody has two identical antigen-binding sites and consists of two light chains and two heavy chains joined by disulfide bonds. The variable regions at the ends of the Y arms form the antigen-binding site, while the constant stems and lower arms form the Fc region which interacts with immune cells and complement proteins.

1. Dr. P. Saranraj M.Sc., M.Phil., Ph.D., NET
Assistant Professor
Department of Microbiology
Sacred Heart College (Autonomous)
Tirupattur, Vellore District.
Mobile: 9994146964
E.mail: microsaranraj@gmail.com
IMMUNOGLOBULINS

2. IMMUNOGLOBULINS
Antibody, also known as an Immunoglobulin (Ig), is a
large, Y-shaped protein (Soluble Glycoprotein) produced
mainly by Plasma cells that is used by the immune
system to neutralize microbial pathogens.
Antibodies are present in the Blood serum,Tissue fluids
and Mucosal surfaces of vertebrate animals.
Each antibody has at least two identical sites that bind to
Antigens.These sites are known as Antigen-binding sites
(or) Paratopes.
The number of Antigen-binding sites on an
antibody is called the Valence of that antibody. For
example, most human antibodies have two binding sites;
therefore, they arc Bivalent.

3. The first use of the term "antibody" occurred in a text
by Paul Ehrlich.
Gerald Maurice Edelman who shared the 1972 Nobel
Prize in Physiology or Medicine for work with Rodney
Robert Porter on the immune system. Edelman's Nobel
Prize-winning research concerned discovery of the
structure of Antibody molecules.
The first evidence that antibodies were contained in
particular serum protein fractions came from a classic
experiment by A.Tiselius and E.A. Kabat in 1939.
Immunoglobulins constitute 20 to 25 % of total serum
protein.

4. STRUCTURE OF IMMUNOGLOBULINS
Gerald Maurice Edelman shared the 1972 Nobel Prize
with Rodney Robert Porter for the discovery of the
structure of Antibody molecules.
A bivalent (arranged in pairs) antibody has the simplest
molecular structure, it is called a Monomer.
Heavy and Light Chains
A typical antibody monomer has 4 protein chains: 2
identical Light chains and 2 identical Heavy chains.
Each Light chain polypeptide usually consists of about 220
amino acids and has a mass of approximately 25,000 Da.
Each Heavy chain consists of about 440 amino acids and
has a mass of about 50,000 to 70,000 Da.

5. Disulfide Bonds
The chains (Light and Heavy chains) are joined by Disulfide
links and other bonds to form aY-shaped molecule.
TheY-shaped molecule is flexible and can assume a T shape.
Variable (V) and Constant Regions (C)
Both Light and Heavy chains contain two different regions.
They are (i)Variable region and (ii) Constant region.
The two sections located at the ends of the “Y” arms are
calledVariable (V) regions.
The Variable region is present in both Light chain (VL) (110
Amino acids) and the Heavy chain (VH) (110Amino acids).
The Variable regions folded together and forms the
Antigen binding site (Paratope).

6. The stem of the antibody monomer and the lower parts of
the arms of the “Y” are called the Constant regions (C).
The Constant region is present in both Light chain (CL) (110
Amino acids) and the Heavy chain (CH) (330 - 440 Amino
acids). Five major types of C regions
There arc, which account for the five major classes of
Immunoglobulins.
Hinge region
The four chains are arranged in the form of a
flexible “Y” with a Hinge region.
The Hinge region allows the antibody molecule to be more
flexible, adjusting to the different spatial
arrangements of Epitopes orAntigenic determinants.

7. Fab region
The top of the Y consists of two Antigen-binding
fragments (Fab) that bind with compatibleAntigens.
Fab regions are composed of bothVariable and Constant
region (MW 45,000 D).
Fc region or Crystallizable Fragments
The stem of the “Y” shaped antibody monomer is called the
Fc region (MW 50,000 D).
Fc regions are important in Immunological reactions.
Fc regions are composed of only Constant region.
Fc region interacts with cell surface receptors
called Fc receptors and some proteins of the complement
system. This property allows antibodies to activate
the immune system.