Immunoglobulin Structure and Function

1. ImmunoglobulinStructure 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 - + albumin Amount of protein globulins γ β α1 α2 Immune serum Ag adsorbed serum Mobility

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 : ץαμδε

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

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 150 Variability Index 100 HVR2 HVR1 50 FR2 FR1 FR4 FR3 0 25 75 100 50 Amino acid residue • 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

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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 Secretory Piece J Chain

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 Y Y Y Y Y Y

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

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) Ig-α Ig-β Ig-β Ig-α

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 Proteinsin 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