ACKNOWLEDGE TO

Asif zaib LECTURER KMU

Structure of Antibodies

Simply Y shape structureComposed of 2 identical heavy polypeptide chains and 2 identical

light chain. Simply 4 polypeptide chains.Heavy chain means have high molecular weight i.e 50,000-77,000d

and composed of 450 amino acid.There are five chain classed of heavy chain i.e mu(u), gamma(r),

alpha(a), delta(&) and epsilon(e)Light chain are smaller having molecular weight 25000 and

composed of 212 amino acidLight chain may be further divide into either kappa or lambda.Immunoglobulin may contain either kappa or lambda but not both

contiLight and heavy chain are join to each other by disulfid bondThe light and heavy chain is composed of constant and variable

region.The first 110-120 amino acid of the light and heavy region form

variable sequence and thought to antigen binding site.The constant region is further divide into C1H,C2H and C3H each

have their own function.C1H: BindsC2H:Complement fixation.C3H:Interectaion between Fc receptors on macrophages and

monocytes.In the medal of antibody a region called hinge region which gives

feasibility to antibody.

Immunoglobulin Fragments: Structure/Function Relationships

Ag Binding

Complement Binding Site

Placental Transfer

Binding to Fc Receptors

Human Immunoglobulin Classes

Despite similarities, Abs are divided (size, charge, solubility) into distinct classes and subclasses

• IgG - Gamma (γ) heavy chains• IgM - Mu (µ) heavy chains• IgA - Alpha (α) heavy chains• IgD - Delta (δ) heavy chains• IgE - Epsilon (ε) heavy chains

Igg• Structure

• Monomer

IgG1, IgG2 and IgG4 IgG3

IgG

• Structure• Properties

• Major serum Ig about 85% of imunoglobulin• Monomeric structure• Can cross placenta as have receptor on placenta• Can Fixes complement (± IgG4)

• Opsonin antibodies and help in phagocytosis.• Incomplete antibodies• Best react at 37 c

Subclass• IgG Subclass

• IgG1 - Gamma 1 (γ1) heavy chains have 2 inter chain disulfide bond

• IgG2 - Gamma 2 (γ2) heavy chains have 4• IgG3 - Gamma 3 (γ3) heavy chains have 11• IgG4 - Gamma 4 (γ4) heavy chains have 2Ig3 have highest ability to activate complement followed by Ig1 and

then Ig2 while Ig4 cant

IgM

• Structure• Pentamer , so it can bind

10 antigen at a time.• Extra domain (CH4)• Also contain J chain which

form disulfiod bond with hinge region

• Diameter 300 Angstrom• Length of monomer is 100

A• 25-30 A width.

Cµ4

J Chain

IgM

•Properties• 3rd highest serum Ig First Ig made by fetus and B cells• It Fixes complement• Can not cross placenta• Complete antibodies• Usual temp 20c but best temperature is 4c–Agglutinating antibody

IgA

• Structure• In Serum 90% occurs as monomer and 10% as dimer.• In Secretions (sIgA) 90% occurs as Dimer and 10% as

monomer.• IgA Have J chain in its structure

Also have Secretory component.

IgAProperties

• It is 2nd highest serum Ig• Major secretory Ig (Mucosal or Local Immunity)

• Tears, saliva, gastric and pulmonary secretions and server as first line of defense against microorganism invasion.

• Does not fix complement• Cannt cross placenta• Temp 37c

•IgA subtype• IgA1 - Alpha 1 (α1) heavy chains• IgA2 - Alpha 2 (α2) heavy chains

IgD

• Structure• Monomer

Tail Piece

IgD• Structure• Properties

• 4th highest serum Ig• B cell surface Ig• Does not bind complement• Function is unknown but may important as lymphocytic differentiation.

IgE

• Structure• Monomer• Extra domain (CH4)

Cε4

IgE• Structure• Properties

• Least common serum Ig• Binds to basophils and mast cells • Does not require Ag binding.• Allergic reactions• Binds to Fc receptor on eosinophils• Does not fix complement

Abs: Molecules for immune effector system

a. Activate C’ – lysis & phagocytosis of microbes b. Binds to Ag – enhance phagocytosis c. Recognition of target d. Stimulates degranulation of mast cells & release mediators e. Cross placental barrier