ANTIGEN & ANTIBODY

Ms. Neha S. RautM.Pharm, PGDPL

Assistant Professor

ANTIGENINTRODUCTION

In 1899 Ladislas Deutsch (Detre) (1874–1939) namedthe hypothetical substances halfway between bacterialconstituents and antibodies

"substances immunogenes or antigenes". "substances immunogenes or antigenes".

He originally believed those substances to be precursorsof antibodies, just like zymogen is a precursor ofzymase. But by 1903 he understood that an antigeninduces the production of immune bodies (antibodies)and wrote that the word antigen was a contraction of"Antisomatogen.

DEFINITION OF ANTIGENAntigen is substance which when introducedparentally into the body stimulates theproduction of an antibody with which it reactsspecifically and in an observable manner

Complete antigen : Substances which can induceantibody formation by themselves and can react specificallywith these antibodies

ClassificationBased on Immunogenicity

Incomplete antigen (haptens): substances unableto induce antibody formation on its own but canbecome immunogenic when covalently linked toproteins, called carrier proteins .they are of two types:

Complex

Simple

BASED ON ORIGIN:

Exogenous antigensExogenous antigens are antigens that have

entered the body from the outside, for example byinhalation, ingestion, or injection. The immunesystem's response to exogenous antigens is oftensystem's response to exogenous antigens is oftensubclinical.

Endogenous antigensEndogenous antigens are antigens that have

been generated within previously normal cells as aresult of normal cell metabolism, or because ofviral or intracellular bacterial infection.

AutoantigensAn autoantigen is usually a normal protein or complexof proteins (and sometimes DNA or RNA) that isrecognized by the immune system of patients sufferingfrom a specific autoimmune disease. These antigensshould, under normal conditions, not be the target ofthe immune system, but, due to mainly genetic andenvironmental factors, the normal immunologicaltolerance for such an antigen has been lost in thesetolerance for such an antigen has been lost in thesepatients.

IsoantigensHeterophile antigen

FACTORS OF ANTIGENECITY

1.FOREIGNNESS

Antigen must be foreignness toimmune system:What substances are foreignness toimmune system ?immune system ?

According to Burnnet’s clone selection

theory , foreignness ( non-self) means

substances that never contact with

lymphocytes during embryo period

2.SIZELarger molecules are highly antigenicLower molecular weight are either non

antigenic or weakly antigenic

3.Chemical Nature3.Chemical NatureMost naturally occurring antigens are either

proteins or polysaccharides.Lipids and nucleic acids are less antigenic on

their own but do so when combined withproteins

4. SUSCEPTIBILITY TO TISSUEENZYMES

Substances which can be metabolised andare able to the action of tissue enzymebehave as antigen.

5.Antigenic specificityIt depends upon epitopePosition of epitope in the antigen molecule

is important for specificity.

6.Species specificityTissue of all individual in species possess species

specific antigens.

7.Isospecificity It depends on isoantigens which may be found in some

but not all members of species.

8.AutospecificitySelf antigens are generally non-antigenic but in some

case such as lens protein and sperm these are notrecognised as self antigen because they are absentduring the embryonic life and develop later.

9.OrganspecificitySome organs such as the brain, kidney and lens

protein of different species share the same antigens.such antigens are the characteristics of an organ ortissue found in different species and they are knownas organ specific antigen.

10.Heterogenic specificity10.Heterogenic specificityThe same or closely related antigen may some time

occur in different biological species, classes andkingdom are know as heterophile antigens.

Antibodies to these antigen produced by one speciescross react with antigen of other species. Eg. ForssmanAg, Weil-felix rxn., Paul-Bunnell test

SUPERANTIGENSWhen the immune system encounters a

conventional T-dependent antigen, only a smallfraction (1 in 104 -105) of the T cell population isable to recognize the antigen and becomeactivated (monoclonal/oligoclonal response).activated (monoclonal/oligoclonal response).

However, there are some antigens whichpolyclonally activate a large fraction of the T cells(up to 25%). These antigens are calledsuperantigens

TESTS FOR ANTIGENDETECTION

Direct ELISADirect ImmunofluoresenceRIARIANeutralization testCFTImmunohistochemistry

DEFINITION:An antibody or immunoglobulin (Ig) is a

ANTIBODIES

An antibody or immunoglobulin (Ig) is aglycoprotein that is made by plasmacells in response to an antigen and canrecognize and bind to the antigen thatcaused its production.

BASIC STRUCTURES:

Antibodies have more than one antigencombining site Some bivalent Ab moleculescan combine to form multimeric Abs that haveupto 10 combining sites

All Ig have a basic structure composed of 4polypeptide chains connected to each other bydisulphide bonds.

Each light chain consist of 220 aa and has amass of approx. 25kDa.

Each heavy chain consists of about 440 aa andhas a mass of 50-70kDa.

Both light and heavy chains contain 2different regions

constant andvariable regionThe four chains are arranged in the form

of a flexible “Y” with the hinge region andof a flexible “Y” with the hinge region andis termed as crystallizable fragment (Fc)and contains the site at which Ab binds.

Top of the “Y” consist of two Ag bindingfragments (Fab) that bind with antigenicdeterminant sites.

LIGHT CHAIN:The light chain may be either of two

distinct forms called “Kappa” and“Lambda” and can be distinguished by aasequence of carboxyl portion of the chain.

HEAVY CHAIN:In the heavy chain NH2 terminal has a

pattern of variability similar to that ofkappa and lambda of the light chain.

BASIC STRUCTURE OF IMMUNOGLOBULIN

IMMUNOGLOBULINS VARIANTS:

om

Immunoglobulin FunctionEach end of the immunoglobulin molecule has a

different role.The Fab region is concerned with binding to antigen,

whereas the Fc region mediates binding to host tissue,various cells of the immune system, some phagocyticcells, or the first component of the complementsystem.system.

The binding of an antibody with an antigen usuallydoes not cause destruction of the antigen or of themicroorganism , cell, or agent to which it is attached.

Rather the antibody serves to mark and identify thetarget for immunologic attack and to activatenonspecific immune responses that can destroy thetarget.

For example:- Bacteria that are covered with antibodies

are better targets for phagocytosis byneutrophils and macrophages.

The alteration of the surface of bacteria,viruses , and other particles so that they canviruses , and other particles so that they canbe more readily phagocytized is termedopsonization.

Immune destruction also is promoted byantibody-induced activation of the classicalpathway.

IMMUNOGLOBULIN CLASSES:

1. IgG:-

FUNCTIONS:

Immunity to new born.Potent activator of complement

system.system.Neutralisation of toxins.IgG3 binds to Fc receptors-

phagocytosis.

2. IgM:

FUNCTIONS:Activation of classical pathway.

Defence against multivalent bacterialDefence against multivalent bacterialantigens.

Acts as opsonin.Present on B-cell surface, acts as antigen

receptor.

3. IgA:-

FUNCTIONS:Secretory antibody.Effective against bacterial like

Salmonella,Vibrio cholerae,Vibrio cholerae,

Niesseria gonorrhoea, etc.Effective against viruses like those

causing Poliomyelitis & Influenza.Protection to infant gut.

4.IgD:

FUNCTIONS:oB cell activation .oActs as receptor for Ag binding.

5.IgE:

FUNCTIONS:oResponsible for immediate hypersensitivity or

allergic reactions.oBinds to Fc receptors on basophils and mast cells.oRelease of substances like histamine , bradykinin

and other vasoactive ‘mediators’.

PROPERTIES OF IMMUNOGLOBULINS:

IgG IgA IgM IgD IgE

1. Serum conc.(%)

85 5-15 5-10 <1 <1

2. Mol. Wt. 160,000 170,000 &385,000

960,000 184,000 188,105

3.Sed. coeff. 7S 7S 19S 7S 8S3.Sed. coeff. 7S 7S 19S 7S 8S

4.Heavy chainclass

Gamma Alpha Mu Delta Epsilon

5.Light chain K & L K & L K & L K & L K & L

6. Valency 2 2 ormultiple of2

5 (10) 2 2

7.No of basic 4-polypeptidechains

Monomer Dimer orTrimer

Pentamer

Monomer Monomer

IgG IgA IgM IgD IgE

8.Placental transport + _ _ _ _

9.Present in milk + + _ _ _

10.Selectie secretion byseromucus gland

_ + _ _ _seromucus gland

11. Intravasculardistribution(%)

45 42 80 75 50

12.Carbohydrate (%) 3 11 10 13 12

8.Subclasses IgG1-4 IgA1-2 _ _ _

IgG IgM IgA IgD IgE

14.Comple-mentfixation

A.Classical ++ _ +++ _ _

B.Alternative

_ + _ _ _

15.Half life(days)

23 6 5 2-3 2-3(days)

16.Princip-al site ofaction

Serum Secretion Serum Receptorfor B cells

Mast cells

17.charact-eristicpropert-ies

precipitins,antitoxins,compleme-nt fixation,late Ab

Serum andsecretoryAbs

Agglutinin,opsonin ,lysin ,early Ab

Not known(B-cellactivation)

ReaginicAb (anaph-ylaxis)

THANK YOU !