1. Immunoglobulins: Structure and Function
Chapter 4
Immunoglobulins: Structure and Function
Oct 17, 2002

2. 本章大綱: Basic structure of immunoglobulins (Ig) 2. Ig sequencing studies
3. Ig fine structure
4. Ig classes and biological activities
5. Antigenic determinants on Ig
6. B-cell receptor
7. The Ig superfamily
8. Monoclonal antibodies

3. Basic structure of immunoglobulins

4. Electrophoresis of serum proteins
(Tiselius & Kabat, 1939)
Immune sera
Removed by Ag

5. g- globulin (gG)
Immunoglobulin (Ig):
IgG, IgM, IgA, IgE, IgD
Antibody (Ab)

6. Basic structure of Ab molecules
IgG: g
IgM: m
IgA: a
IgD: d
IgE: e

7. Basic structure of Ab molecules

8. Papain digestion of an IgG molecule
(Porter, 1950s)
mw 150 kDa
Fab fragment: antigen binding
mw 45 kDa
Fc fragment: crystallizable
mw 50 kDa

9. Pepsin digestion of an IgG molecule
(Nisonoff, 1950s)
mw 100 kDa
(multiple fragments)

10. Mercaptoethanol reduction
of an IgG molecule
mw 25 kDa
mw 50 kDa

12. Antibody to the Fab fragment could react with both the H and L chains, whereas antibody to the Fc fragment reacted only with the H chain.
 Fab consists of portions of an H and a L chain.
 Fc contains only H chain components.

13. Immunoglobulin sequencing studies

14. - A heterogeneous spectrum of antibodies
in the serum g-globulin fraction
Multiple myeloma, a cancer of Ab-
producing plasma cells
Myeloma protein, 95% of the serum Ig
Bence-Jones proteins, the excess light chains
in the urine.
MOPC, mineral-oil induced plasmacytoma
in mice

16. Immunoglobulin Fine Structure

17. Immunoglobulin Domains Variable-Region Domains
- hypervariable (HV) regions:
(15% - 20% of the variable domain)
- framework regions (FR)
- complementarity-determining regions (CDR)
CDR1, CDR2, CDR3
Constant-Region Domains
Hinge Region

19. Hypervariable
Region

20. Complementarity-
determining regions

22. Immunoglobulin Classes Biological Activities
and
Biological Activities

23. 5 major classes of secreted antibody

24. IgG
most abundant in serum
80% of total serum Ig
4 IgG subclasses

25. 4 subclasses of human IgG
size of the hinge region
no. & position of the interchain
-S-S- bond
- IgG1>IgG2>IgG3>IgG4 in serum conc.
90% - 95% homologous in DNA sequences

26. IgM Pentamer - monomer on the membrane & pentamer in secretion
- 5% - 10% serum Ig
- 1st Ab in neonates
- 1st Ab in primary
response
- more efficient in
agglutination & C fixation
- J (joining) chain allows
IgM to transport across
the epithelium.
- also present in secretions

27. IgA Dimer 10% - 15% of total serum Ig monomers, dimers, trimers and
tetramers in serum
- predominant in external secretions,
e.g., breast milk, saliva, tears, and
mucus of the bronchial, genitourinary,
and digestive tracts

28. Secretory IgA
Dimers and tetramers in secretion with a secretory component

29. IgE potent biological activity extremely low conc. in serum
- mediates the immediate hyper-
sensitivity reactions
responsible for the symptoms
of hay fever, asthma, hives, and
anaphylactic shock

30. IgD 0.2% of total serum Ig - together with IgM, is the
major membrane-bound Ig
on mature B cells
thought to function in the
activation of B cells
no biological effector
function has been identified

31. - Antibodies generally do not kill or remove
pathogens solely by binding to them.
While V regions bind to Ag, the CH region
is responsible for a variety of collaborative
interactions with other proteins, cells, and
tissues that result in the effector functions
of the humoral responses.

32. Ig-Mediated Effector Functions
- Opsonization
- Activation of complement (chapter 13)
- Antibody-Dependent Cell-Mediated
Cytotoxicity (ADCC )
- Ab transport through epithelium or placenta
- Activation of mast cells, eosinophils and
basophiles by IgE (chapter 16)

33. Opsonization the promotion of phagocytosis of Ags by
Mf and neutrophiles
- Fc receptors (FcR)

34. Activation of Complement

35. C1        C9
Chapter 13

36. Activation of Complement
(membrane
attack
complex)

37. Antibody-dependent Cell-mediated Cytotoxicity (ADCC)

38. Transcytosis - movement of Ab across epithelial layer
- delivery of IgA to the mucosal surfaces of the
respiratory, gastrointestinal, and urogenital tracts,
as well as its export to breast milk
- transfer of IgG from mother to fetus
(passive immunization)

39. Formation of Secretory IgA
Transcytosis

40. IgA in the Gut

41. Secretory IgA in Breast Milk
Bind to microbes in baby’s digestive tract and
thereby prevent their attachment to the walls of
the gut and their subsequent passage into the
body’s tissues.

42. Allergen cross-linkage of receptor-bound
IgE on mast cells induces degranulation

44. Antigenic Determinants on Immunoglobulins

45. 3 Antigenic Determinants
of Immunoglobulins
Isotypes
Allotypes
Idiotypes

46. Isotypic Determinants
constant-region determinants that collectively define
each H-chain class and subclass, and each L-chain type
and subtype within a species

47. Allotypic Determinants
differences in amino acids in C regions, which occur
in some, but not all, members of a species

48. Idiotypic Determinants
The unique amino acid sequence of the V regions of a given Ab.
In some cases an idiotype is the actual antigen-binding site.

49. B-Cell Receptor

50. B-cell Receptor

51. The Immunoglobulin Superfamily

52. The Immunoglobulin Superfamily (1)

53. The Immunoglobulin Superfamily (2)

54. Monoclonal Antibodies

55. Clonal Selection of B Lymphocytes