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Chapter 4. Immunoglobulin Structure
and Function
**1. Functional Regions
- Types of chains
- Constant &** **Variable regions
- Glycoprotein**
- Each heavy and light chain is made up of a number of domains (= Ig folding or Ig domains).
- Light chains consist of 2 domains (C and V).
- Heavy chains have 4- domains (depending on the class of antibody)
- Each domain is about 110 amino acids in length and contains an intrachain disulfide bond between two cysteines about 60 amino acids apart.
Heavy chain= 446 aa Light chain= 214aa
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1
2
2
- 150,000 molecular weight
- Constant (C) and Variable (V) regions
What is the difference?
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2 3 4
Basic Antibody Structure
• Multiple myeloma = cancerous plasma cells
• Monomer = 150,
2 Fab + Fc
2 H + 2 L
(Fab) 2
100,000 MW
Papain
Pepsin
Mercaptoethanol
RECAP:
- The Fc region plays NO role in antigen binding.
- Papain breaks antigen molecules into 2 Fab fragments and an Fc fragment.
- Pepsin breaks antibody molecules into an F(ab’) 2 fragment and a VERY SMALL pFc’ fragment.
- Mercaptoethanol treatment results in 2 heavy and 2 light chains
- Complexes of antibodies cross-linked by antigen are called “immune complexes”.
Figure 3.
- Constant region - amino acid sequence in the C- terminal regions of the H and L chains is the same.
- Variable region - amino acid sequence in the N- terminal regions of the H and L chains is different. This region provides antibodies with unique specificity.
- Hyper-variable regions are regions within the variable regions (greater specificities).
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1
2
3
Summary
- Molecule consists of Constant and Variable regions for both Light and Heavy chains (CH, VH, CL, VL)
- Ig molecule made of domains
- Domains ~ 110 aa
- Each antigen-binding site is made up of the N- terminal domain of the heavy and the light chains
- IgM and IgE possess 4 CH domains (CH1-CH4) while IgG, IgA and IgD have 3 CH domains (CH1- CH3). Hinge region is missing.
- Hypervariable regions in the Variable regions of both H and L chains.
-Within the variable domains are three regions of extreme variability.
These are referred to as the hypervariable regions.
These regions of the variable domains actually contact the antigen.
They therefore make up the antigen-binding site.
These regions are also called the complementarity- determining regions, or CDRs.
Heavy Chain Light Chain
Complementarity-Determining Regions, or CDRs.
Structural Variants of the Basic Immunoglobulin Molecule
Different heavy chains can be used There are five major types of heavy chain --> five major classes (isotypes) of antibody
- gamma --> IgG (in humans 4 subclasses: IgG1, IgG2, IgG3, IgG4)
- mu --> IgM
- alpha --> IgA (in humans, 2 subclasses: IgA1, IgA2)
- delta --> IgD
- epsilon --> IgE
The function of antibody varies depending on which heavy chain is used.
IgG IgM IgA IgD IgE
Relative abundance in normal serum:
IgG 8 - 16 mg/ml
IgA 1.4 - 4 mg/ml IgM 0.5 - 2 mg/ml
IgD 0.003 - 0.04 mg/ml
IgE 17 - 450 ng/ml (<0.0005 mg/ml)
IgG
IgA
IgM
IgD IgE
IgG IgA IgM IgD IgE
- Most abundant in secondary responses
- Crosses placenta (FcRn)
- Complement activation
- Binds to FcR in phagocytes
Figure 3.15a
Crosses placenta Crosses placenta Crosses placenta Complement Activator Complement Activator Complement Activator Fc binding Fc binding (^) - Best Complement activation
- First Ab produced in neonate
- First antibody produced after challenge
- Mucosal transport (to some degree)
- Monomer on B cells
- J chain: polymeric
- Dimer in mucosal secretions
- **Mucosal transport
- Monomer in circulation
- J chain (polymeric) and Secretory components**
Secretory Component
Role of IgE in allergic reactions
IgE antibodies mediate the immediate- hypersensitivity (allergic) reactions that are responsible for symptoms of hay fever, asthma, hives and anaphylactic shock.
IgE binds to Fc receptors on the membranes of blood basophils and tissue mast cells.
Cross-linkage of receptor-bound IgE molecules by antigen (allergen) induces degranulation of basophils and mast cells.
A variety of pharmacologically active mediators present in the granules are released, giving rise to allergic manifestations
IgD
**- Role unknown
- Present on the** surface of MATURE B cells Marker!!
- IgA and IgM are secreted across epithelial surfaces
- IgG, IgD and IgE can be found only within the body - in serum or lymph.
- IgA and IgM are also found in serum and lymph BUT IN ADDITION can also be found in secretions such as mucous secretions, saliva and tears.
- The IgA and IgM found in external secretions differs from that found in serum by the presence of an additional component referred to as the "secretory component".
- This component is acquired as the IgA or IgM is transported across the epithelial cell barrier.
SUMMARY (^) Antigenic Determinants on
Immunoglobulins
- Abs are glycoproteins and themselves very immunogenic
- Epitopes on immunoglobulins are divided into: - ISOTYPIC - ALLOTYPIC - IDIOTYPIC
Receptors
Neonatal
Immune Function
Monoclonal Antibodies
• Kohler & Milstein 1975
• Fusion of normal, activated B cell and
plasmacytoma (cancerous plasma cell)
• Hybrid: immortal, secrete Ab, hypoxanthine
Plasmacytoma VS B cell
• Plasmacytoma:
- Cancerous plasma cell (Immortal)
- Does not secrete Abs
- Lacks HGPRT
• Normal spleen B cell
- Limited life span
- Secretes Abs
- Possess HGPRT
RESULTS:
Spleen B cell Hybrid Plasmacytoma
Die in culture Immortal, Secretes Lacks HGPRT Ab, Possess hypoxanthine (HGPRT)
Applications?
• Diagnosis
• Research
• Treatment
• Affinity VS Avidity
Affinity (polyclonal Ab) = high because
of multiple epitopes
Avidity (monoclonal Ab) = low affinity but
high avidity because of strong epitope-
Ab interaction
The End
IgG - Most abundant Ig of internal body fluids (serum, extracellular fluids) - combats microorganisms and toxins within the body tissues. IgA - Most abundant Ig in mucous secretions - protects external surfaces of the body IgM - The first class of antibody produced during an immune response. Present both in internal body fluids and in secretions. IgD - Functions not well defined. Found mostly on the B cell plasma membrane IgE - Increases during parasitic infections. Causes symptoms of allergy.
Binds tomacrophages +++ + - - + and polymorphs
Binds to mastcells and - - - - +++ basophils
Ability to crossthe placenta ++ - - - -
Complementfixation by ++ - +++ - - classical pathway
IgG IgA IgM IgD IgE
