Chapter 4 Immunoglobulins: Structure and Function

1. 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. Complementarity- determining regions

21. Immunoglobulin Classes and Biological Activities

22. 5 major classes of secreted antibody

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

24. 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

25. 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

26. 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

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

28. 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

29. 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

30. - 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.

31. 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)

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

33. Activation of Complement C1

34. C1        C9 Chapter 13

35. Activation of Complement (membrane attack complex)

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

37. Transcytosis • - movement of Ab across epithelial layer • - delivery of IgA to the mucosal surfaces of the • respiratory,gastrointestinal, andurogenitaltracts, • as well as its export tobreast milk • - transfer of IgG from mother to fetus • (passive immunization)

38. Formation of Secretory IgA Transcytosis

39. IgA in the Gut

40. 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.

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

43. Antigenic Determinants on Immunoglobulins

44. 3 Antigenic Determinants of Immunoglobulins Isotypes Allotypes Idiotypes

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

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

47. 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.

48. B-Cell Receptor

49. B-cell Receptor

50. The Immunoglobulin Superfamily