Immunoglobulins- Serum proteins are against a number of important bacterial infections.IntroductionAll vertebrates possess immunoglobulin-like molecules.Immunoglobulins are synthesized and secreted by end cells of the B cell lineage, i.e. plasma cell.Immunoglobulins are largely confined to the broad and heterogeneous band of -globulins and show considerable diversity of structure and function.

The first evidence to show antibody is one of serum protein fractions1939, A. Tiselius and E. A. KabatTo immunize rabbits with ovalbuminBelong to -globulin fractionimmunoglobulin

Structure of ImmunoglobulinsThe typical immunoglobulin molecule isAsymmetrically composed of four polypeptide chainsLinked by disulphide bridgesThe larger chains are designated heavy(MW about 50~77 kDa) and the smaller light (MW about 25 kDa).The two most important of their features in the immune response are specificity and biologic activity. Several immunoglobulin fragments can be prepared using proteolytic enzymes and these have been of value in unravelling the functional activities of different parts of these molecules

Schematic diagram of structure of immunoglobulins

Papain digestionTo cleave on the amino-terminal side of the inter-heavy chain disulphide bonds.To yield two FabFragment antigen-bindingfragments and one Fc Fragment crystallizablefragment.Pepsin digestion To cleave on the carboxy-terminal side of the inter-heavy chain disulphide bonds.To yield a Fab dimerF(ab' )2and a rather smaller Fc fragmentpFc'

The immunoglobulin molecules are in Y configuration and have the flexible hinge region which permits considerable movement of the Fab arms.Both light and heavy polypeptide chains consist of a series of similar subunits, designated domains. domain110 amino acidsa single intrachain disulphide bridgepolypeptide chainLight and heavy polypeptide chainsamino-terminal domainvariable region()domainconstant region()light chaindomains(VLCL)heavy chaindomains (VHCH1CH2CH3CH4domainimmunoglobulin)

Ribbon representation of an intact monoclonal antibody depicting the heavy and light chain

Qutternary structure immunoglobulin and interactions between domainsExtended peptide sequence between CH1 and CH2 of , , and heavy chainPoline rich sequenceGiving IgG, IgD and IgA segmental flexibility

Variable regionimmunoglobulin(antigen-binding site)hot spots (hypervariable regionsHVR1HVR2HVR3 or complementarity-determining regions, CDR1CDR2CDR3) Affinity: the strength of binging or association of a single epitope for a single combining siteAvidity: indicate the average strength of binding

Diagram of an immunoglobulin light chain depicting the fold structure Held teogether by hydrophobic interaction and disulfide bond

Antigenic Deteminants on ImmunoglobulinAntibodies also are potent immunogens to induce an antibody responseAnti-Ig antibodies are powerful tools Antigenic determinants (epitopes) on Ig:Isotypic determinant: constant-region determinants, to distinguish each Ig class and subclass within a speciesAllotypic determinant: be able to distinguish the subtle amino acid difference between the same set of isotype genes Idiotypic determinant: be generated by conformation of amino acid sequences of heavy- and light-chain variable regions specific for each antigen

Individual determinant is called an idiotope.The sum of the individual idiotope is the idiotype

The Classes and Subclasses of Immunoglobulins heavy chain constant regionImmunoglobulinsIgGIgAIgMIgDIgEIgGIgAsubclasses

IgGIgDIgEmonomerIgAdimerIgMpentamerJ chainA small acidic protein15 kDalight and heavy chainplasma cellJ chain is disulfide-bonded to the penultimate cysteine residue in the tail piece of theorchainimmunoglobulin monomerSecretory componentA single polypeptideabout 70 kDaIgAIgAsecretory componentcarbohydrate J chainimmunoglobulinsIgA

IgG 1234 heavy chainslight chainIgG1(60-70%)IgG2(14-20%)IgG3(4-8%)IgG4(2-6%)IgG accounts for about 75-80% of the total serum immunoglobulin in normal adult and is the abundant antibody produced during secondary humoral immune responses in the bloodIgG1IgG2IgG3classical complement pathwayIgG2alternative complement pathway

IgM heavy chainlight chainpentamer10 antigen-binding sitesJ chainconstant regionsIgM accounts for only about 10% of serum immunoglobulinis the key immunoglobulin of the primary response and an efficient activator of the classical complement pathwayand is the most common immunoglobulin expressed on the surfaces of B cellspoly-Ig recoptorIgA deficiencysecretory pieceIgA

IgA 12heavy chainslight chainIgA1IgA2(51)dimer (secretory IgA, pentamer)secretory componentJ chainmembrane-bound formmonomer

IgA accounts for only about 10-15% of serum immunoglobulinThe major immunoglobulin of external secretionsThe most abundant antibody class was found in saliva, tear, intestinal mucus, bronchial secretions, milk, prostatic fluid, and other secretionsclassical complement pathwayalternative complement pathwayC3 convertasestabilizationSpecific Fc receptors for IgA have been observed but are not well charaterized

Formation of secretory IgA:Dimer IgA binds to a poly-Ig receptorInternalized by receptor-mediated endocytosisPloy-Ig receptor is enzymatically cleavedSecretory component bound to the dimer IgA

IgE heavy chainlight chainabout 180 kDaIg(0.004%)allergic disorder()constant regionsmast cellsbasophilshigt-affinity Fc receptor (FcRI)inflammatory mediators(such as eosinophils)FcRIIIgE()parasitic worms

Allergen cross-linkage of receptor-bound IgE on mast cell induces degranulation, causingrelease of substances that mediate allergic manifestations

IgD heavy chainlight chainBIgDIgMBantigen receptor