1. Immunoglobulin Structure and Function

2. Immunoglobulins: Structure and Function
Definition: Glycoprotein molecules that are produced by plasma cells in response to an immunogen and which function as antibodies
Immune serum
Ag adsorbed serum α1 α2 β γ + -
albumin
globulins
Mobility
Amount of protein

3. ANTIBODIES
Immunoglobulins : Proteins of animal origin endowed with antibody activity
Immune sera
Gamma globulins
All antibodies are immunoglobulins, but
all immunoglobulins may not be antibodies
Immunoglobulins : Structural & Chemical concept
Antibodies : Functional concept
Immunogloblins : 20 – 25 % of total serum proteins
Five classes of Immunoglobulins : IgG, IgA, IM, IgD, IgE

4. STRUCTURE Cleavage study : Fc and Fab fractions
Glycoproteins consisting of 2 pairs of polypeptide chains
L chains (Light) & H chains (Heavy)
L chains : Mol wt 25,000, Attached to heavy chains by a disulphide bond
- Occur in 2 varieties : 1. Kappa (Ќ)
2. Lambda (λ)
H chains : Mol wt 50,000, Two h chains joined together
by 1-5 SS bonds
- 5 classes of H chains : ץ α μ δ ε

5. Immunoglobulin Fragments: Structure/Function Relationships
Ag Binding
Complement Binding Site
Placental Transfer
Binding to Fc Receptors

6. Immunoglobulin Fragments: Structure/Function Relationships
Fab
Ag binding
Valence = 1
Specificity determined by VH and VL
Papain Fc
Fab Fc
Effector functions

7. Immunoglobulin Fragments: Structure/Function Relationships
Fab
Ag binding Fc
Effector functions
F(ab’)2
Pepsin
Fc Peptides
F(ab’)2

11. Antigen combing site : Aminoterminus
Fc fragment site : Carboxy terminal for other biological activities
- Complement fixation
- Placental transfer
- Skin fixation & catabolic rate
Variable and Constant regions
Hypervariable regions (Hot spots) : CDR’s (Complementarity
determining regions)
Fd piece : portion of H chain in Fab fragment
Globular domains : Inter chain disulphide bonds form loops in the peptide chain and lops are compactly folded, having specific functions
- Vl and VH for formation of specific antigen binding site
- CH2 binds C1q complement
- CH3 mediates adherence to monocyte surface
Hinge region : Flexible exposed to enzymes an chemicals

12. Structure of the Variable Region
Hypervariable (HVR) or complimentarity determining regions (CDR)
HVR3
FR1
FR2
FR3
FR4
HVR1
HVR2
Variability Index 25 75 50
100
Amino acid residue
150
Framework regions

13. Human Immunoglobulin Classes
IgG - Gamma (γ) heavy chains
IgA - Alpha (α) heavy chains
IgM - Mu (µ) heavy chains
IgD - Delta (δ) heavy chains
IgE - Epsilon (ε) heavy chains

14. Human Immunoglobulin Subclasses
IgG Subclasses
IgG1 - Gamma 1 (γ1) heavy chains
IgG2 - Gamma 2 (γ2) heavy chains
IgG3 - Gamma 3 (γ3) heavy chains
IgG4 - Gamma 4 (γ4) heavy chains
IgA subclasses
IgA1 - Alpha 1 (α1) heavy chains
IgA2 - Alpha 2 (α2) heavy chains

16. +

18. IgG Major serum immunoglobulin – 80%
Molecular wt 1,50,000 (7S) , occasionally in polymersed form
Distributed equally between intravascular an extra vascular
Half life : 23 days
Serum concentration : 8 to 16 mg per ml
Transported across placenta
Binds to microoranisms and enhances phagocytosis
General purpose antibody active in blood and tissues
Participates in Complement fixation, Precipitation and neutralization of toxins and viruses
Four sub classes : IgG1, IgG2, IgG3, IgG4
- Serum conc : 65%, 23%, 8%, 4% resp

20. IgG
Structure
Monomer (7S)
IgG1, IgG2 and IgG4
IgG3

21. IgA Structure Serum - monomer Secretions (sIgA) Dimer (11S) J chain
Secretory component
J Chain
Secretory Piece

22. IgA 10 to 13% of serum immunoglobulin, 0.6 to 4.2 mg/ml
Major Ig in colostrum, saliva and tears, Half life 6-8 days
1. Serum IgA – Monomeric 7S (Mol wt 1,60,000)
2. Secretory IgA : Dimeric SIgA , 11 S (4,00,00 mol wt)
J chain : IgA found on mucosal surfaces and in secretions is a dimer joined by 2 monomer units joined together at carboxy terminals by a glycoprotein
Secretory component : Glycine rich polypeptide produced by mucosal cells, protect IgA from denaturation by bacterial proteases
Antibody paste, inhibit adherence of microorganisms to mucosal surface, promotes phagocytosis, activate alternate Complement pathway
Two sub class : IgA1 & IgA2

24. Origin of Secretory Component of sIgA

26. IgM
Cµ4
J Chain
Structure
Pentamer (19S)
Extra domain (CH4)
J chain

27. IgM 5- 8% of serum Ig, 0.5 – 2mg/ml, Half life 5 days
Millionaire molecule : 19 S, 9,00,000 mol wt
Polymers of 5 subunits with J chain
Oldest and earliest, short lived Ig, 80% intravascular
Produced by foetus at 20 weeks of age
Iso haemagglutinins, Natural abs to micro organisms, Abs to Typhoid O Ag, Reagin Abs in syphilis
Monomeric IgM is the major antibody receptor on surface of B lymphocytes for antigen recognition
A single IgM molecule bring about Immune haemolysis, thousand times more effective in opsonisation,100 times effective in bactericidal action and 20 times in agglutination

28. B Cell Antigen Receptor (BcR)

29. IgD
Structure
Monomer
Tail piece
Tail Piece

30. IgD Resembles IgG structurally 3 mg / 100 ml conc in serum
Mostly intravascular
Half life 3 days
Occur on the surface of B lymphocytes and serve as recognition receptors for antigens

31. IgE
Structure
Monomer
Extra domain (CH4)
Cε4

32. IgE Atopic reagin antibodies, Ishizaka 1966
8 S molecule, Molecular wt 1,90,000, Half life 2 days
Extravascular in distribution
Elevated levels in atopic conditions like, asthma, hay fever, eczema and parasitic infections
Chiefly produced in linings of respiratory and intestinal tracts
Responsible for Anaphylactic type of hypersensitivity
Protect against pathogens by mast cell de-granulations and release of inflammatory mediators

34. ABNORMAL IMMUNOGLOBULINS
Structurally similar proteins to antibodies in serum
Seen in many pathological processes
Bence Jones Proteins in Multiple myeloma : Light chains of immunoglobulins, either kappa or lambda
- Identified in urine by coagulation when heated to 50 0C but redissolving at 70 0 C
- Multiple myeloma may affect Ig G,A,D,E
Waldenstrom’s macrogluobulinemia : Myeloma of IgM producing plasma cells – Excessive production of M proteins
Cryoglobulinemia : Formation of a gel or precipitate on cooling the serum, which redissolves on warming
- Associated with myelomas, macroglobulinaemias and autoimmune conditions such as systemic lupus erythematosus

35. Immunoglobulin specificities
Idiotopes : Specific antigenic determinants on paratope
Idiotype : Sum total of idiotopes on Ig molecule
Antiidiotypic antibodies : Produced by immunization with Fab fragments, resembles epitopes of original antigen
Used as vaccines to protect against pathogen or tumour

36. Isotypic and Allotypic specificities
Genetically determined specificities based on their antigenic structure
Isotypic specificity : Antigenic specificity which distinguish between the different classes and subclasses of Ig’s present in all normal individuals of a given species
Allotypic specificity : Antigenic specificity which distinguish Ig’s of the same class between different groups of individuals in the same species
Anti-allotype specific abs may develop following blood transfusion or passage of maternal IgG into foetus and also seen in sera containing RA factor.
2 Allotype systems in humans : 1. Gm system 2. InV system
Gm is associated with Fc portion of IgG heavy chain, more than 25 Gm types identified so far
InV associated with kappa light chain (Km) and 3 Km types
Am : Genetic markers associated with IgA