Chapter 4Immunoglobulins: Structure and Function

Oct 17, 2002

本章大綱 :

1. Basic structure of immunoglobulins (Ig)2. Ig sequencing studies3. Ig fine structure4. Ig classes and biological activities5. Antigenic determinants on Ig6. B-cell receptor7. The Ig superfamily8. Monoclonal antibodies

Basic structure of immunoglobulins

Electrophoresis of serum proteins(Tiselius & Kabat, 1939)

Immune sera

Removed by Ag

globulin (G)

Immunoglobulin (Ig): IgG, IgM, IgA, IgE, IgD

Antibody (Ab)

Basic structure of Ab molecules

IgG: IgM: IgA: IgD: IgE:

Basic structure of Ab molecules

Papain digestion of an IgG molecule

(Porter, 1950s)

mw 150 kDa

Fab fragment: antigen binding mw 45 kDa

Fc fragment: crystallizable mw 50 kDa

Pepsin digestion of an IgG molecule(Nisonoff, 1950s)

mw 100 kDa

(multiple fragments)

Mercaptoethanol reduction of an IgG molecule

mw 50 kDamw 25 kDa

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.

Immunoglobulin sequencing studies

- A heterogeneous spectrum of antibodies in the serum -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

Immunoglobulin Fine Structure

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

Complementarity-determining regions

Immunoglobulin Classes and

Biological Activities

5 major classes of secreted antibody

IgG

- most abundant in serum

- 80% of total serum Ig

- 4 IgG subclasses

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

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

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

Secretory IgA

Dimers and tetramers in secretion with a secretory component

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

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

- Antibodies generally do not kill or remove pathogens solely by binding to them.

- While V regions bind to Ag, the CCH H regionregion is responsible for a variety of collaborative interactions with other proteins, cells, and tissues that result in the effector functions of the humoral responses.

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)

Opsonization

- the promotion of phagocytosis of Ags by Mand neutrophiles

- Fc receptors (FcR)

Activation of Complement

C1

C1 C9

Chapter 13

(membrane attack complex)

Activation of Complement

Antibody-dependent Cell-mediated Cytotoxicity (ADCC)

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)

Formation of Secretory IgA

Transcytosis

IgA in the Gut

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.

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

Antigenic Determinants on Immunoglobulins

3 Antigenic Determinantsof Immunoglobulins

Isotypes

Allotypes

Idiotypes

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

Allotypic Determinants

- differences in amino acids in C regions, which occur in some, but not all, members of a species

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.

B-Cell Receptor

B-cell Receptor

The Immunoglobulin Superfamily

The Immunoglobulin Superfamily (1)

The Immunoglobulin Superfamily (2)

Monoclonal Antibodies

Clonal Selection of B Lymphocytes