Immunoglobulin Structure and Function

ImmunoglobulinImmunoglobulin

Structure and FunctionStructure and Function

Immunoglobulins: Structure and Immunoglobulins: Structure and FunctionFunction

• 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

Am

oun

t of

pro

tein

ANTIBODIESANTIBODIES

• Immunoglobulins Immunoglobulins : Proteins of animal origin endowed with : Proteins of animal origin endowed with antibody activityantibody activity

• Immune seraImmune sera• Gamma globulinsGamma globulins• All antibodies are immunoglobulins, but All antibodies are immunoglobulins, but

all immunoglobulins may not be antibodiesall immunoglobulins may not be antibodies• ImmunoglobulinsImmunoglobulins : Structural & Chemical concept : Structural & Chemical concept• AntibodiesAntibodies : Functional concept : Functional concept• ImmunogloblinsImmunogloblins : 20 – 25 % of total serum proteins : 20 – 25 % of total serum proteins• Five classes of Immunoglobulins : Five classes of Immunoglobulins : IgG, IgA, IM, IgD, IgEIgG, IgA, IM, IgD, IgE

STRUCTURESTRUCTURE

• 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: Immunoglobulin Fragments: Structure/Function RelationshipsStructure/Function Relationships

Ag Binding

Complement Binding Site

Placental Transfer

Binding to Fc Receptors

Immunoglobulin Fragments: Immunoglobulin Fragments: Structure/Function RelationshipsStructure/Function Relationships

• Fab– Ag binding– Valence = 1– Specificity

determined by VH and VL

Papain

Fc

Fab

• Fc– Effector functions

Immunoglobulin Fragments: Immunoglobulin Fragments: Structure/Function RelationshipsStructure/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 RegionStructure of the Variable Region

• Hypervariable (HVR) or complimentarity determining regions (CDR)

HVR3

FR1 FR2 FR3 FR4

HVR1HVR2

Var

iabi

lity

Ind

ex

25 7550 100Amino acid residue

150

100

50

0

• Framework regions

Human Immunoglobulin ClassesHuman 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 Human Immunoglobulin SubclassesSubclasses

• 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

+

IgGIgG• 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

IgGIgG

• Structure– Monomer (7S)

IgG1, IgG2 and IgG4 IgG3

IgAIgA

• Structure– Serum - monomer– Secretions (sIgA)

• Dimer (11S)

• J chain

• Secretory component

J ChainSecretory Piece

IgAIgA• 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 sIgAOrigin of Secretory Component of sIgA

IgMIgM

• Structure– Pentamer (19S)– Extra domain

(CH4)

– J chainCµ4

J Chain

IgMIgM

• 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)B Cell Antigen Receptor (BcR)

Ig-αIg-β Ig-βIg-α

IgDIgD

• Structure– Monomer– Tail piece

Tail Piece

IgDIgD

• 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

IgEIgE

• Structure– Monomer– Extra domain

(CH4)

Cε4

IgEIgE

• 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 IMMUNOGLOBULINSABNORMAL 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 specificitiesImmunoglobulin 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

• Genetically determined specificities based on their antigenic structure

• Isotypic specificity : 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 : 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

Isotypic and Allotypic specificities