Structure and Function of Antibodies

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IMMUNOGLOBULINS
STRUCTURE & FUNCTIONS
Presented by,
B.GURUPREETHA,
III- B.Tech (Biotechnology),
Vivekanandha College of Engineering for Women.

3. Definition
 Glycoprotein molecules that are produced by plasma cells in response
to an immunogen and which function as antibodies.
 Y-shaped protein used by the immune system to identify and
neutralize foreign objects such as pathogenic bacteria and viruses.
 The immunoglobulins derive their name from the finding that they
migrate with globular proteins when antibody-containing serum is
placed in an electrical field.

4. Background
 In 1890, von Behring and Kitasato Found a agent in the blood that
could neutralize diphtheria toxin.
 In 1939, Tiselius and Kabat used electrophoresis to separate
immunized serum
 albumin
 alpha-globulin
 beta-globulin
 gamma-globulin fractions

5. Contd.,
 Absorption of the serum against the antigen yielding the terms
gamma globulin, immunoglobulin (Ig), and IgG.

6. Contd.,
 Sizing columns were then used to separate immunoglobulins
into those that were
 “heavy” (IgM),
 “regular” (IgA, IgE, IgD, IgG), and
 “light” (light chain dimers).

7. Role of different
immunoglobulin classes
 IgG: Protects the body fluids
 IgA: Protects the body
surfaces
 IgM: Protects the blood stream
 IgE: Mediates type I
hypersensitivity
 IgD: Role not known

8. Basic Structure

9. Contd.,
 1. Heavy and Light Chains
 All immunoglobulins have a four
chain structure as their basic unit.
They are composed of two identical
light chains (23kD) and two identical
heavy chains (50-70kD)

10. Contd.,
2.Hinge regions:
 This is the region at which the arms of the
antibody molecule forms a Y.
 It is called the hinge region because there is
a some flexibility in the molecule at this
point.

11. Contd.,
3. Disulfide bonds
i) Inter-chain disulfide bonds -
The heavy and light chains and the two
heavy chains are held together
ii) Intra-chain disulfide binds -
Within each of the polypeptide chains
there are also intra-chain disulfide bonds.

12. Contd.,
4. Variable (V) and Constant (C) Regions
 Two regions based on variability in the amino
acid sequences
1. Light Chain - VL (110 amino acids) and
CL (110 amino acids)
2. Heavy Chain - VH (110 amino acids) and
CH (330-440 amino acids)

13. Contd.,
5. Domains
Folded globular regions containing
intra-chain disulfide bond are called
domains.
i. Light Chain Domains - VL and CL
ii. Heavy Chain Domains - VH, CH1 -
CH3 (or CH4)

14. Contd.,
6. Oligosaccharides
 Carbohydrates are attached to the CH2
domain in most immunoglobulins.
However, in some cases carbohydrates
may also be attached at other
locations.

15. Overall
diagram

16. Structure of the variable
region
A. Hypervariable (HVR) or complementarity
determining regions (CDR)
 CDR is the antibody combining site
 Antibodies with different specificities (i.e. different
combining sites) have different complementarity
determining regions while antibodies of the exact
same specificity have identical complementarity
determining regions.

17. Contd.,
B. Framework regions
 The regions between the complementarity determining
regions in the variable region are called the framework
regions
 This regions can be divided into groups and subgroups.
These represent the products of different variable region
genes.

18. Contd.,

19. IMMUNOGLOBULIN
FRAGMENTS
Immunoglobulin fragments produced by proteolytic digestion
have proven very useful in elucidating structure/function
relationships in immunoglobulins

20. Contd.,
A. Fab
 This results in the formation of two identical fragments that contain the light
chain and the VH and CH1 domains of the heavy chain.
 Antigen binding - These fragments were called the Fab fragments because they
contained the antigen binding sites of the antibody.
 The combining site of the antibody is created by both VH and VL.
 An antibody is able to bind a particular antigenic determinant because it has a
particular combination of VH and VL.

21. Contd.,
B. Fc
 This fragment was called Fc because it was easily crystallized.
 Effector functions - The effector functions of immunoglobulins are
mediated by this part of the molecule.
 Different functions are mediated by the different domains in this
fragment.

22. Contd.,
C. F(ab')2
 This fragment was called F(ab')2 because it is divalent.
 The Fc region of the molecule is digested into small peptides by
pepsin.
 The F(ab')2 binds antigen but it does not mediate the effector
functions of antibodies.

23. Functions
 Ag binding
 Can result in protection
 Valence
 Effector functions (Usually require Ag binding)
 Fixation of complement
 Binding to various cells

24. Antigen binding

25. Antibody effector functions

26. Summary

27. References:
 Diane F. Jelinek et al. Middleton’s 8th Edition Abbas 9th Edition.
 Schroeder, Harry & Cavacini, Lisa. (2010). Structure and Function of
Immunoglobulins. The Journal of allergy and clinical immunology. 125.
S41-52. 10.1016/j.jaci.2009.09.046.
 Peter, Spaeth. (1999). Structure and Function of Immunoglobulins. Sepsis.
3. 197. 10.1023/A:1009899803032.
 J Allergy Clin Immunol. 2010 Feb; 125(2 0 2): S41–S52. doi:
10.1016/j.jaci.2009.09.046

28. Thank you!