Antibodies, also known as immunoglobulins, are Y-shaped glycoprotein molecules produced by plasma cells in response to antigens. They belong to a class of proteins called immunoglobulins. The arms of the Y bind antigens while the tail is responsible for biological activity. Antibodies are composed of two light chains and two heavy chains that give them different structures and functions. The five major classes of antibodies are IgG, IgA, IgM, IgD, and IgE, which differ in size, structure, concentration in serum, and roles in the immune response.

1. Antibodies: Structure And
Function

2. • Glycoprotein molecules that are produced by plasma cells in
response to an immunogen and which function as antibodies.
• Antibodies belong to a class of proteins called
immunoglobulins
• Immunoglobulins are “Y” shaped proteins. The“arms” of the
“Y” bind antigens. The tail of the “Y” is responsible for
biological activity
Immune serum
Ag adsorbed serum

1

2
 
+ -
albumin
globulins
Mobility
Amountofprotein

3. General Functions of Immunoglobulins
• Ag binding
– Can result in protection
– Valency
• Effector functions
– Fixation of complement
– Binding to various cells
– Usually requires Ag binding

4. Basic Immunoglobulin Structure
• Immunoglobulins - heterogeneous
• Myeloma proteins - homogeneous
immunoglobulins

5. Antibody Structure
• Antibodies Are Made Up Of:
– 2 Light Chains (identical) ~25 KDa
– 2 Heavy Chains (identical) ~50 KDa
• Each Light Chain Bound To Heavy Chain By Disulfide
(H-L)
• Heavy Chain Bound to Heavy Chain (H-H)
• First 100 a/a Of Amino Terminal Vary of Both H and L
Chain Are Variable
• Referred To As VL , VH, CH And CL
• CDR (Complementary Determining Regions) Are What
Bind Ag
• Remaining Regions Are Very Similar Within Same
Class

7. • Repeating Domains of ~110 a/a
– Intrachain disulfide bonds within each domain
• Heavy chains
– 1 VH and either 3 or 4 CH (CH1, CH2, CH3, CH4)
• Light chains
– 1 VL and 1 CL
• Hinge Region
– Rich in proline residues (flexible)
– Hinge found in IgG, IgA and IgD
– Proline residues are target for proteolytic digestion (papain and
pepsin)
– Rich in cysteine residues (disulfide bonds)
– IgM and IgE lack hinge region
– They instead have extra CH4 Domain
Antibody Structure

9. • Digestion With Papain Yields
– 3 Fragments
– 2 identical Fab and 1 Fc
– Fab Because Fragment That is Antigen Binding
– Fc Because Found To Crystallize In Cold Storage
• Pepsin Digestion
– F(ab`)2
– No Fc Recovery, Digested Entirely
• Mercaptoethanol Reduction (Eliminates Disulfide Bonds)
And Alkylation Showed
Enzymatic Digestion Of Antibodies

11. • Sequencing Of Several Immunoglobulins Revealed
– 100-110 Amino Terminus, Highly Variable (V)
– Five Basic Sequence Patterns
– , , , , 
– IgA, IgD, IgG, IgE and IgM
– The Above Classes Are Called Isotype
– Each class can have either  or  light chains
– Minor differences Led To Sub-classes For IgA and IgG
– IgA1, IgGA2 and IgG1, IgG2, IgG3, IgG4
Sequencing Of Heavy Chains

12. Characteristics of hinge region
•Immunoglobulins (with possible exception of IgM & IgE)
contain hinge between
• CH1 & CH2
•No homology between AA sequence of hinge & heavy chains
•AA sequence differs with different classes
•Comprised of many cysteine and proline residues
•Cysteine involved in formation of interchain disulfide bonds
•Proline prevents folding in a globular structure, allowing
flexibility between
two Fab arms of the Y-shaped antibody; allows open & close to
accommodate
binding to two epitopes; because it is open, it can be cleaved by
proteases
(e.g. papain) to generate the Fab & Fc fragments

13. Immunoglobulin Fragments:
Structure/Function Relationships
• Fab
– Ag binding
• Fc
– Effector functions
• F(ab’)2
Pepsin
Fc
Peptides
F(ab’)2

14. Immunoglobulin Fragments:
Structure/Function Relationships
• Fab
– Ag binding
– Valence = 1
– Specificty
determined by VH
and VL
Papain
Fc
Fab
Fc
Effector functions

15. Immunoglobulin Fragments:
Structure/Function Relationships
Ag Binding
Complement Binding Site
Placental Transfer
Binding to Fc
Receptors

16. IgG
IgG1 IgG2
IgG3 IgG4
Subclasses show close overall relation
Heavy chains = 1,2,3,4
Highest concentration in serum
Plays major role in immune dfns.
Small size ppt in surfaces
(e.g. cross placental barrier)
All normal indiv. have all
IgG1>IgG2>IgG3>IgG4
IgG3 has shortest 1/2 life,
highest catabolic rate,
highest # S-S
IgG4 = monoval. no aggl
ppt rx., autoab to clot fac
Autoab to DNA = IgG1,IgG3
Two  H chains, Two L chains
(either  or  but not both)
Each H chain = 50 kD
Each L chain = 25 kD
MW approx. 150 kD, 7S, “ globulin”

17. IgG
• Structure
– Monomer (7S)
IgG1, IgG2 and IgG4 IgG3

18. IgG
• Structure
• Properties
– Major serum Ig
– Major Ig in extravascular spaces
– The only antibody to cross the placental
– Fixes complement
– Binds to Fc receptors
• Phagocytes - opsonization
• NK cells – ADCC
– Binds to SPA

19. IgM
• Structure
• Properties
– 3rd highest serum Ig
– First Ig made by fetus and B
cells
– Produced early in the
primary response
– The most efficient Ig
– Fixes complement
Tail
Piece
Agglutinating Ig
Binds to Fc receptors
B cell surface Ig

20. IgM
• Structure
– Pentamer (19S)
composed 5
H2L2 units plus one
molecule of
J chain
– Extra domain (CH4)
– J chain
CH4
J Chain

21. Structural features of IgM
Pentamer (5)
First Ig produced following immun.
Macroglobulin (M)
900 kD, 19S
SSS S
SS
J chain Doesn’t have hinge region
has additional H domain
Has a J chain (one of 2 Ig isotypes)
15 kD

22. IgD
• Structure
– Monomer
– Tail piece
Tail Piece

23. IgD
• Structure
• Properties
– 4th highest serum Ig
– B cell surface Ig
– Does not bind complement

24. Structural features of IgD
Primarily a B cell antigen receptor
Long exposed hinge region
170kD, 7S, migrates as fast 
No interchain S-S bridges in H chains
Readily denatured

26. IgA
• Properties
–2nd highest serum Ig
–Major secretory Ig ( saliva, tears,
respiratory, intestinal, and genital
tract secretions.)
–Does not fix complement unless
aggregated
–Binds to Fc receptors on some cells

27. IgA
• Structure
–Serum - monomer
–Secretions (sIgA)
–Dimer (11S), sIgA molecule consists of two
H2L2 units plus one molecule each of J chain
and secretory component(SC or SP)
J ChainSecretory Piece

28. IgE
• Structure
– Monomer
– Extra domain (CH4)
CH4

29. Structural features of IgE
Sometimes called reaginic ag
Mediates allergies (Type I
hypersensitivies)
190 kDa, 8S, migrates as fast 
Contains an extra domain (CH4)
which binds to mast cells
& basophils
May remain attached for long time
when ag reappears, cross links
IgE on mast cell surface, release
mast-cell granules & signs of
anaphylaxis

30. IgE
• Structure
• Properties
– Least common serum Ig
– Allergic reactions
– Parasitic infections
– Does not fix complement

31. Fixation of C1 by IgG and IgM Abs
No activation Activation

32. B Cell Antigen Receptor (BcR)
Ig-Ig- Ig-Ig-

33. L & H chain folding to yield 3 CDR in each
chain to form walls of ag binding groove