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					    Antibodies

Immunoglobulins (Ig)


    Dr.Ghada Boghdadi
Objectives:

* Explain the basic immunoglobulin structure and its
   relationship with different functions.

* Describe the structural & functional differences among the
   major Immunoglobulin class.

* Explain the role each immunoglobulin class plays in an
   immune response.

* Explain the genetic basis for the generation of antibody
   diversity.

* Discuss monoclonal antibodies.



                                             Anantha Narayanan
                                                         Ch 15
Ig:

* glycoprotein molecules that are produced by plasma cells in
response to an immunogen.


*      Introduction of an Ag into an animal



      Abs appeared in the serum & body fluids



* Abs react with Ag in a specific & observable manner.
* Electrophoresis of human serum: albumin and globulin (,  & )

*  globulin fraction of serum proteins = antibodies (Igs)

* 5 classes of Igs – IgG, IgA, IgM, IgD & IgE.

* Constitutes 20-25% of total serum proteins.
GENERAL FUNCTIONS OF IMMUNOGLOBULINS:


A. Antigen binding

B. Effector function.

                 -   Fixation of complement
                 -   Binding to various cell types
BASIC STRUCTURE OF
IMMUNOGLOBULINS:



* 2 identical light (L) chains (kappa () &
lambda ()) & 2 identical heavy
(H) chains which are distinct for each of
the five Ig classes and are designated
, , , ,   and   .
Disulfide bonds: inter and
intra chain bonds.

* Variable (V) and Constant
(C) Regions.

* Hinge region.

* Domains
   STRUCTURE OF THE VARIABLE REGION:



The variable regions of
both L and h chains have three
extremely variable (hypervariable)
(the complementarity determining
Regions) amino acid sequences
at the amino-terminal end that form
the antigen-binding sites.
IMMUNOGLOBULIN FRAGMENTS: STRUCTURE/FUNCTION RELATIONSHIPS




  1- Fab

  2- Fc

  3- F(ab’)2
HUMAN IMMUNOGLOBULIN CLASSES:



  1- IgG - Gamma heavy chains 

  2. IgM - Mu heavy chains 

  3. IgA - Alpha heavy chains   
  4. IgD - Delta heavy chains 

  5. IgE - Epsilon heavy chains 
Immunoglobulin Subclasses:

   based on small differences in the amino acid sequences
   in the constant region of the heavy chains:

1. IgG Subclasses

a) IgG1 - Gamma 1   heavy chains
b) IgG2 - Gamma 2   heavy chains
c) IgG3 - Gamma 3   heavy chains
d) IgG4 - Gamma 4   heavy chains




2. IgA Subclasses

a) IgA1 - Alpha 1 heavy chains
b) IgA2 - Alpha 2 heavy chains
IgG

Structure: Monomer
Percentage serum antibodies: 80%
Location: Blood, lymph, intestine
Half-life in serum: 23 days
Complement Fixation: Yes
Placental Transfer: Yes
Known Functions: Enhances phagocytosis,
neutralizes toxins and viruses, protects fetus and
newborn.
IgA
Structure: Dimer
Percentage serum antibodies: 10-15%
Location: Secretions (tears, saliva,
intestine, milk), blood and lymph
Half-life in serum: 6 days
Complement Fixation: No
Placental Transfer: No
Known Functions: Localized protection of
mucosal surfaces. Provides immunity to
infant digestive tract
Origin of Secretory Component of
               sIgA
IgM


Structure: Pentamer
Percentage serum antibodies: 5-10%
Location: Blood, lymph, B cell surface (monomer)
Half-life in serum: 5 days
Complement Fixation: Yes
Placental Transfer: No
Known Functions: First antibodies produced during
an infection. Effective against microbes and
agglutinating antigens
IgD
Structure: Monomer
Percentage serum antibodies: 0.2%
Location: B-cell surface, blood, and lymph
Half-life in serum: 3 days
Complement Fixation: No
Placental Transfer: No
Known Functions: In serum function is
unknown. On B cell surface, initiate
immune response
IgE
Structure: Monomer
Percentage serum antibodies: 0.002%
Location: Bound to mast cells and
basophils
Half-life in serum: 2 days
Complement Fixation: No
Placental Transfer: No
Known Functions: Allergic reactions
Possibly lysis of worms
Antibody diversity:

One of the major questions in immunology has been how can we make so
many different antibody molecules.


Ab diversity is due to the Ig genes.

One or only few genes code for C region whereas many genes code for the V
region.

The V region of each L chain is encoded by two genes segments ( V + J ).
The V region of each H chain is encoded by three genes segments( V + J +D).


http://outreach.mcb.harvard.edu/animations/antibody.swf
Monoclonal antibodies:

antibodies of a single specificity that are
all built alike because they are being manufactured by a single clone of plasma
cells that can be grown indefinitely.

				
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