Antibodies or ImmunoglobulinsAntibodies or Immunoglobulins

** Definition:Definition: Glycoprotein in serum and tissue fluidGlycoprotein in serum and tissue fluid ** Produced by:Produced by: B-lymphocytes in response to exposure to antigenB-lymphocytes in response to exposure to antigen

** React specifically with antigenReact specifically with antigen

* Five classes of Antibodies:* Five classes of Antibodies: IgGIgG IgMIgM IgAIgA IgDIgD IgEIgE

IgGIgG

PropertiesProperties• Major serum IgMajor serum Ig• Major Ig in extravascular spacesMajor Ig in extravascular spaces• The only Placental transfer Ig The only Placental transfer Ig • Fixes complement Fixes complement • Phagocytes - opsonizationPhagocytes - opsonization

IgMIgM

PropertiesProperties• First Ig made by fetus and B cellsFirst Ig made by fetus and B cells• Present in colostrum and mother Present in colostrum and mother

milk protect newly born.milk protect newly born.• Fixes complementFixes complement

IgAIgA --Found in Found in serum and body secretion:serum and body secretion: Tears, saliva, gastric and pulmonary Tears, saliva, gastric and pulmonary secretions secretions

• Major secretory Ig on Mucous surfacesMajor secretory Ig on Mucous surfaces give Local Immunity by coating m.o, give Local Immunity by coating m.o, bacteria or viruses preventing their bacteria or viruses preventing their adherence to mucosal cellsadherence to mucosal cells

• Does not fix complement (unless Does not fix complement (unless aggregated)aggregated)

• Present in Present in colostrum and mother milkcolostrum and mother milk protect newly born.protect newly born.

IgEIgE

• Least common serum IgLeast common serum Ig Binds to basophils and mast cells (Does not Binds to basophils and mast cells (Does not

require Ag binding)require Ag binding)• Allergic and hypersensitivityAllergic and hypersensitivity reactions reactions• Parasitic infections (Helminths)Parasitic infections (Helminths)

Binds to Fc receptor on eosinophilsBinds to Fc receptor on eosinophils• Does not fix complementDoes not fix complement

IgDIgD

• Present in very small amount in serumPresent in very small amount in serum• B cell surface IgB cell surface Ig• Does not bind complementDoes not bind complement

Antibody ProductionAntibody Production Clonal selection theory:Clonal selection theory:

**B-cells displayB-cells display immunoglobulin molecules on immunoglobulin molecules on surfacessurfaces

*Immunoglobulin serve as *Immunoglobulin serve as receptors for specific antigenreceptors for specific antigen

*The *The antigenantigen binds to binds to immunoglobulin receptor of B-cellsimmunoglobulin receptor of B-cells

**B-cellsB-cells is stimulated to divide and form is stimulated to divide and form a clone a clone

**B-cellsB-cells become become plasma cellsplasma cells and secrete and secrete antibodiesantibodies

*Some *Some stimulated B-cellsstimulated B-cells revert to revert to small lymphocyte (memory cells)small lymphocyte (memory cells)

*Memory cells proliferate on re-exposure to same antigen *Memory cells proliferate on re-exposure to same antigen

Activation of B-cellsActivation of B-cellsTwo mechanisms:Two mechanisms: 1) T-dependent antigen1) T-dependent antigen:: . Most . Most antigenantigen require require T-helper cellsT-helper cells to activate to activate B-cellsB-cells

. Antigen is phagocytosed by . Antigen is phagocytosed by macrophagesmacrophages or or B-cellsB-cells

. . Macrophages or B processMacrophages or B process present present AgAg to to T-cellsT-cells

. These activate . These activate T-cellsT-cells to produce to produce lymphokines lymphokines

. lymphokines (IL-2,IL-4,IL,5) stimulate . lymphokines (IL-2,IL-4,IL,5) stimulate B-cells B-cells to to divide and differentiate into plasma cellsdivide and differentiate into plasma cells

specific antibodyspecific antibody . Plasma cells form . Plasma cells form oror differentiate into memory cellsdifferentiate into memory cells . All classes of antibody (IgG,IgM,IgA,IgD,IgE) are T-cell dependant. All classes of antibody (IgG,IgM,IgA,IgD,IgE) are T-cell dependant

Activation of B-cellsActivation of B-cells

2) T-independent antigens2) T-independent antigens::

.. Activation of Activation of B-cells B-cells directly without directly without help of T-cellshelp of T-cells

(e.g. bacterial capsular polysaccharides)(e.g. bacterial capsular polysaccharides)

.. IgM antibody is primarily produced IgM antibody is primarily produced

Antibody StructureAntibody Structure ImmunoglobulinsImmunoglobulins are glycoproteins made up ofare glycoproteins made up of - Four polypeptid chains (IgG): - Four polypeptid chains (IgG): a-a- Two light (L)Two light (L) polypeptide chainspolypeptide chains b- Two heavy (H) polypeptide chainsb- Two heavy (H) polypeptide chains - The four chains are linked by disulfide bonds- The four chains are linked by disulfide bonds

- Terminal portion of L-chain contains part of antigen binding site - Terminal portion of L-chain contains part of antigen binding site

- H-chains are distinct for each of the five immunoglobulins- H-chains are distinct for each of the five immunoglobulins

- Terminal portion of H-chain participate in antigen binding site- Terminal portion of H-chain participate in antigen binding site

- The other (Carboxyl) terminal portion forms Fc fragment- The other (Carboxyl) terminal portion forms Fc fragment

ANTIBODY STRUCTUREANTIBODY STRUCTUREAn antibody molecule is composed of two identical Ig heavy chains (H) and two identical light chainslight chains (L), each with a variable region (V) & constant region (C).

Amino acid sequences were determined from myeloma proteins.Fig. 1-17,1-16

Variable(V) and Constant (C) Regions - Each- Each HH-chain-chain and eachand each LL-chain-chain hashas V-V-regionregion andand C-C-regionregion - - V-V-regionregion lies in terminal portion of molecule lies in terminal portion of molecule

- - V-V-regionregion shows wide variation in amino a. sequencesshows wide variation in amino a. sequences

- - HypervariableHypervariable region form region complementary to Ag determinant region form region complementary to Ag determinant

- It is responsible for antigen binding- It is responsible for antigen binding

- - C-C-regionregion lies in carboxyl or terminal portion of molecule lies in carboxyl or terminal portion of molecule

- - C-C-regionregion shows an unvarying amino acid sequenceshows an unvarying amino acid sequence

- It is responsible for biologic functions- It is responsible for biologic functions

Antibody FragmentsAntibody Fragments Fab fragment:Fab fragment: antigen binding site antigen binding site

Fc (crystallizable fragment):Fc (crystallizable fragment): a- Complement fixation (IgM and IgG)a- Complement fixation (IgM and IgG) b- Opsonization (IgG)b- Opsonization (IgG) C- Placental attachment (IgG)C- Placental attachment (IgG) d- Mucosal attachment (IgA)d- Mucosal attachment (IgA) e- Binding to mast cells (IgE)e- Binding to mast cells (IgE)

Properties of ImmunoglobulinsProperties of Immunoglobulins

PropertyProperty IgG IgG

IgAIgA IgMIgM IgEIgE IgDIgD

Heavy chain Heavy chain symbolsymbol

γγ αα µµ εε δδ

Molecular Molecular weightweight

150 150 KDaKDa

170-400 170-400 KDaKDa

900900 KDaKDa

190190 KDaKDa

180180 KDaKDa

PercentagePercentage in serumin serum

75 %75 % 15 %15 % 10 %10 % 0.004 %0.004 % % 0.2 % 0.2

ComplementComplement fixationfixation

YesYes NoNo YesYes NoNo NoNo

Transplacental Transplacental passagepassage

YesYes NoNo NoNo NoNo NoNo

OpsonizationOpsonization YesYes NoNo NoNo NoNo NoNo

Antibody DiversityAntibody Diversity *Immunoglobulins*Immunoglobulins are protein (antigenic) are protein (antigenic)

*Immunoglobulins*Immunoglobulins subdivided into : subdivided into :

a-a- Isotypes:Isotypes: Antigenic Antigenic difference in C-regiondifference in C-region five immunogl. classes are different isotypesfive immunogl. classes are different isotypes

b-b- Idiotypes:Idiotypes: Ag difference in Ag difference in V-region of immunogl.V-region of immunogl.

c-c- Allotypes:Allotypes: Antigenic feature of immunogl. that Antigenic feature of immunogl. that vary vary among among individualindividual under under genetic controlgenetic control Ag difference in Ag difference in C-region of H and L chainC-region of H and L chain

Primary and Secondary antibody responsePrimary and Secondary antibody response

Primary antibody respone Secondary antibody responsePrimary antibody respone Secondary antibody response

* first exposure to antigen * Subsequent exposure* first exposure to antigen * Subsequent exposure

* lag period: days or weeks * Lag period: hours* lag period: days or weeks * Lag period: hours (slow onset) (rapid onset)(slow onset) (rapid onset) * Small amount immunogl. * large amount immunogl.* Small amount immunogl. * large amount immunogl. low Ab level with gradual increase high Ab with rapid increaslow Ab level with gradual increase high Ab with rapid increas * Ab Persist for short duration * * Ab Persist for short duration * Persist for long periodsPersist for long periods

Weeks then decline rapidly (monthes or years)Weeks then decline rapidly (monthes or years)

* Antibody is * Antibody is IgM IgM * Antibody is * Antibody is IgGIgG

Primary and Secondary antibody responsePrimary and Secondary antibody response

Amount of antibodies in serum

Time (months)1 2 3 4 5 6

1st injection of Ag 2nd injection of Ag

Primary response to Ag

Secondary response to Ag