Immunoglobulin Structure and Function

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

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

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

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

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

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

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

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

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

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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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

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

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

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

Origin of Secretory Component of sIgA

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

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

B Cell Antigen Receptor (BcR)

IgD Structure Monomer Tail piece Tail Piece

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

IgE Structure Monomer Extra domain (CH4) Cε4

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

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

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

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