An antibody, or immunoglobulin, is a glycoprotein produced by the immune system that detects antigens, consisting of four polypeptide chains (two light and two heavy). Each antibody has variable and constant regions responsible for antigen binding and biological functions, classified based on amino acid sequences. Immunoglobulins can be fragmented through proteolysis, yielding functional parts such as Fab for antigen binding and Fc for binding to cell receptors.

1. Structure
of
Antibody
Sijo.A
B.sc.Botany &
Biotechnology
Mar ivanios college
Tvm, Kerala

2. What is Antibody?
 Antibody(Ab) or immunoglobulin(Ig) is the large Y
shaped glycoprotein produced by the body’s immune
system when it detects harmful substances are called
antigens.
 They are synthesized by B lymphocytes and secreted
by plasma cells.
 Depending on the electrophoretic migration, 3 types of
globulins are present in the blood, namely α, β and γ
 So antibodies are gamma (γ) globulin.

3. Structure of Antibody
 The structure of antibody was discovered by Rodney.R.porter
and Gerald Edelman in 1962.
 All antibodies have a common basic structure.
 Each antibody molecule has 4 polypeptide chains.
 Two small called light chain(L) and two longer called heavy
chain(H). Hence an antibody represented as H2L2.
 Each polypeptide chain contains an (NH3+) amino terminal
and carboxy (COO-) terminal.
 They are linked by disulfide bonds.
 L-chains have a molecular weight of 20-25kd.
 The central region of heavy chain is called hinge region and it
is rich in proline.
 Proline prevents the peptide chains from assuming an alpha-
heix conformation.

4. Variable & constant region
 L & H chains are further divided into variable &
constant region.
 These regions are composed of 3D folded repeating
segments are called domains. Each domain contains
110 amino acids.
 The variable region concern with antigen binding and
constant region concern with various biological
functions like complement activation and binding to cell
surface receptors.
 The light chain consists of one variable(vL) & one
constant region(cL).
 The heavy chain consists of one variable (vH) & three
constant (cH) regions.

5. Hypervariability region or
CDR
 The variable region of both L and H contains 3 variable
loops for binding antigens. These antigen binding site is
called CDR (Complementary Determining Region).
 The CDR determines the specificity & affinity of antigen.
 Immunoglobulins are classified on the
basis of amino acid sequence in the constant region of
light chain. They are kappa(k) and lamda(Λ).
 Immunoglobulins are again classified on the basis of
aminoacid sequence in the constant region of heavy
chain. They are:
i) Ig G - γ ii) Ig A - α
iii) Ig M - μ iv) Ig E - ε
v) Ig D - δ

6. Immunoglobulin fragments
 Immunoglobulin fragments are produced by proteolytic digestion.
 The papain (proteolytic enzyme of papaya) digestion of Ig G
molecules produced by 3 fragments. Out of which 2 fragments
are identical and it has antigen binding capacity. So it is termed
as Fab.
 The antigen binding part of antibody is called Fab.
 The third fragment has no Ag binding capacity and observed that
it crystalize during cold storage. So it termed as Fc.
 Fc is a crystalisable fragment that binds to various cell receptors
and complement proteins.
 The pepsin digestion of Ig G molecules
produced 2 Fab like segments & it is designated as F(ab)’2. it
has antigen precipitating capacity & it is stronger than one
combination.
 But Fc fragment is not recovered from pepsin digestion, because
it digested into multiple fragments.


8.  THANK YOU…