1. Antibodies

2. Antibodies (Immunoglobulins)
1. Introduction
2. Structure
3. Functions
4. Overview of the 5 classes of Igs
5. Antibody diversity
6. Antibody preparation

3. 1. Introduction Antibodies
A group of glycoproteins produced by B cells, which bind antigens with high specificity, often termed “immunoglobulins”.
1-1. Distribution
1-2. Heterogeneity

5. Antibody response

6. 1-1. Distribution of antibodies
Serum and tissue fluids
Mediating the humoral immune response
Surface of B cells
Acting as B cell antigen receptor (BCR)

7. Surface and secreted antibodies

8. 1-2. Heterogeneity of antibodies
 to  fractions of normal serum
many classes and types
specific for an antigen epitope

9. 2. The structure of antibodies
2.1. Basic structure of antibody
2.2. Antibody fragments
2.3. Structure of 5 classes of antibodies

10. 2-1. Basic Structure of Antibody (antibody unit)
Basic sructure of Ab is a 4-chain unit
Variable region and constant region
Immunoglobullin domains

12. Basic sructure of Ab is a 4-chain unit
2 identical light chains (L chain)
2 identical heavy chains (H chain)
The L and H chains are linked by disulphide bonds, forming a Y-shaped molecule
2 kinds of L chain: , , forming 2 types of antibodies
5 kinds of H chain: , , ,  and , forming 5 classes of antibodies.

13. Structure of IgG

14. 5 classes of human Abs
depending on the structure of the heavy chain

15. Variable region and constant region
All chains have one variable and one constant region.
V region
The amino-terminal part of the chain, consisting of 100–110 amino acids, are found to vary among different antibodies
V region can be divided into hypervariable region (complementarity determining region, idiotypic determinant) and framework region
C region
The carboxyl-terminal part of the molecule, called the constant (C) region

16. Some functions of the domains
Domains of Antibody
A disulphide bond enclosed peptide loop
L chain: VL and CL
H chain: VH, CH1, CH2. CH3 (IgG, IgA, IgD; IgM and IgE have got an extra CH4)
Some functions of the domains
VL and VH: binding an antigenic epitope specificaly
CH2: binding the C1q of complement.
CH3 (or CH4 of IgM and IgE): binding to the Fc receptor on effector cells.

17. The basic structure of IgG1

18. 2-2. Antibody fragments
Antibody can be proteolytically cleaved to yield to fragments
Fab fragment (fragment that binds antigen)
antigen binding part of antibody
Fc fragment (fragment that crystallized)
effector part of antibody

20. 2-3. Structure of 5 classes of antibodies
IgG, IgE, IgD and serum IgA are of a typical four-chain structure (monomer).
IgM is a pentamer of the basic four-chain units (composed of 5 four-chain units), these units are linked by a peptide called J chain.
Secretory (sIgA) is a dimer of two basic four-chain units, these units are linked by the J chain, A peptide called secretory component is also needed for sIgA traverses epithelial cell layers.

22. 3. Functions of antibodies
Antibodies are bifunctional molecules
3.1 V region
binding to antigen (epitope) specifically
3.2 C region
effector functions

23. 3-1. Functions of V region
Variable regions account for unique antigen binding specificities
Antibody on B cell surface (BCR)
B cells recognize antigen (epitope) specifically
Antibody in serum and interstitial fluid
Neutralizing bacterial toxin, preventing viral attachment to host cells

25. Constant regions mediate different effector functions
3-2. Functions of C region
Constant regions mediate different effector functions
Complement activation (classical pathway)
Binding to Fc receptors on effector cells
- ADCC
- Opsonization
- Type I hypersensitivity
Other functions
- placental transfer, epithelial transfer

29. Antibody dependent cellular cytotoxicity (ADCC)
PMNs, monocytes, macrophages, NK cells, eosinophils can kill antibody-coated target cells directly.
Fab of Ab binds the antigen of the target cell, and the Fc of Ab binds to the FcR on the effector cells, mediating the lysis of the target cells.

31. Opsonization
Process of making a microbe easier to phagocytose. Antibody coating the microbe will facilitate the phagocytes to engulf it.
Fab of Ab binds the antigen of the antigen, and the Fc of Ab binds to the FcR on the phagocytes, facilitating the phagocytosis of the antigen

33. IgE mediating type I hypersensitivity
The Fc of IgE binds to FcR on mast cells and basophils, and the Fab binds antigen, mediating the hypersensitivity

35. 4. Overview of the 5 classes of antibodies
IgG
IgM
IgA
IgD
IgE

36. IgG The major Ig in serum and tissue fluid, 70-75% of total Igs
IgG is a monomer of a 4-chain unit
The predominant Ig produced in secondary response
The most important Ig involved in elimination of pathogens and mediation of inflammation (complement activation, opsonization, ADCC)
Placental transfer, IgG is the only Ig class which is of placental transport to fetus

37. IgM
10% of the total Ig pool, in serum (intravascularly)
A pentamer of 4-chain units, with the largest molecular weight
The predominant Ig in primary response.
Associated with immune response to antigenically complex, blood-borne infectious organisms (A potent activator of complement)

38. IgA
15-20% of the total Ig, 2 forms of IgA (serum IgA and secretory IgA)
Serum IgA is a monomer of 4-chain unit, secretory IgA is a dimer of 4-chain units
The secretory IgA is the predominant Ig seromucous secretions (saliva, colostrum, milk, tracheobronchial and genitourinary secretions)
involved in seromucous defense

39. IgD
1% of the serum Ig, but a major Ig on B cells
A monomer of a 4-chain unit
biological function remains unclear, but BCR
IgE
Scarce in serum, but on the surface of basophils and mast cells
involved in immunity against some parasites, and mediating the allergic diseases

40. 5. Antibody diversity
Isotype: the genetic variants are present in all individuals of a species, e.g. Ab classes and types.
Allotype: the genetic variation between individuals within a species.
Idiotype: the variation in the viable domain, particularly in hypervariable regions, eg. Abs produced in an individual can bind a range of antigens specifically.

42. 6. Preparation of antibodies
Ployclonal antibody
conventional antibody, obtained from an animal immunized with antigen (with many epitopes), mixture of antibodies produced by many B cell clones
Monoclonal antibody (mAb)
antibody of a single specificity (monospecific), produced by a B cell clone specific for an epitope.
Antibody engineering