Download 1-Overview of Opportunistic Infection

ANTIGEN (IMMUNOGEN)
Dr Sateesh.K
Assoc Professor
Slide 1
ANTIGEN (IMMUNOGEN)
Definition: Any substance capable of provoking the
immune system to respond by generating an immune
reaction specifically directed at the inducing substance.
The response may involve humoral or the
cellular limb of the immune response or both.
Slide 2

Definition: Any substance which, when introduced into
the body, stimulates the production of an antibody with
which it reacts specifically and in an observable
manner.
Slide 3
ANTIGEN
Two attributes of Antigenicity
- Induction of immune response (Immunogenicity)
- Sp. reaction with Ab or sensitized cells (Immune
reactivity)
Slide 4
Types of Antigens
Based on the ability to perform functions antigens are
classified as
i.
Complete Antigen
ii.
Incomplete Antigen/Hapten
Complete antigen is able to induce antibody formation
and produce a specific and observable reaction with
antibody.
Hapten (to fasten)

Haptens are substances which are incapable of inducing
antibody formation by themselves, but can react
specifically with antibodies.

Haptens become immunogenic on combining with a larger
molecule carrier.

Types of haptens: Simple & Complex

Simple haptens are monovalent, hence can’t precipitate
with antibody

Complex haptens are polyvalent, hence can precipitate
with antibody
ANTIGEN
Complete Antigen:
Any substance that is able to induce antibody formation
and produce a specific observable reaction with the
antibody so produced.
Hapten (partial antigens):
Low molecular substances incapable of inducing
antibody formation but can react specifically with
antibody. Haptens become immunogen with carrier mol.
(Eg; penicillin)
Slide 7
Slide 8
CLASSIFICATION OF ANTIGENS
Based on origin:
Microbial: Fimbrial, somatic, flagellar, capsular
Based on Immune response:
‘T’ dependant Antigens
Depend on the help of ‘T’ lymphocytes
Eg; serum precursors, Erythrocytes, Haptens etc.
IgM, IgG IgA IgE are the Abs
Long memory
‘
Slide 9
CLASSIFICATION OF ANTIGENS
T’ Independent Antigens
Produce Ab without the help of ‘T’ lympho
Eg; Bact.LPS, Pneumococci polysaccharide,
Fimbrial and flagellar Ags
IgM is the Ab
Large mol wt-Ag with repeating epitopes
Short memory
Slide 10
Slide 11
TERMS COMMONLY USED
Epitope: (Antigenic determinant)
Small area on Ag, capable of sensitizing an
immunocyte and also reacting with complimentary site
on the sp. antibody.
Paratope:
Combining area of the Ab molecule corresponding to
the epitope (Hyper variable region of Ab mol.)
Slide 12
Antigenic determinant

The smallest unit of antigenicity is known as antigenic
determinant or epitope

The epitope is a small area on antigen possessing a
specific chemical nature, electrical charge and steric
configuration, capable of sensitising a immunocyte to
produce antibody and reacting with it.

The combining area on the antibody molecule,
corresponding to the epitope is called as paratope
Determinants of antigenicity
1.
Size
2.
Chemical nature
3.
Susceptibility to tissue enzymes
4.
Foreignness
5.
Specificity
Size
Larger molecular weight substances are more antigenic,
while smaller molecular weight substances are not
antigenic.
Larger mol. (Haemocyanin)
- Mol. < 10,000

More antigenic
Nonantigenic
Low molecular weight substances can be made
antigenic by adsorbing them on large inert particles like
bentonite and kaolin
Chemical nature:
Most naturally occurring antigens are proteins and
polysaccharides. Lipids and nucleic acids are less
antigenic.
- Protein, polysaccharides
- Lipid, nucleic acid
More antigenic
Less antigenic
- Structural complexity; protein with more amino acids,
polysaccharides with more monosaccharides
- Presence of Aromatic amines (Tyrosin)
Slide 16
Susceptibility to tissue enzymes:
Only substances which are metabolised and are
susceptible to action of tissue enzymes behave as
antigens.
 Polysterene
latex is not antigenic
Foreignness
Only nonself (foreign) antigens induce an immune
response. Antigenicity of a substance is related to degree
of its foreignness
 Antigens
from related species are less antigenic than
those of distant species.
- Tolerance to self Ag is conditioned by contact with them
during developmental stage
- Break down
Auto immune disease
- Evolutionarily, greater the gap more is the antigenicity.
Slide 18
DETERMINANTS OF ANTIGENICITY (CONTD.)
Genetic constitution of host
Epitopes:
More the number of epitopes they are more antigenic
Dosage, route and timing of Ag administration
Antigenicity can be enhanced by mixing with an
adjuvants (Substance which release slowly and
maintain the continuous stimulation)
Slide 19
Specificity of natural tissue Ag may be of various types. They
are
I)
Antigenic Specificity
II) Species Specificity
II) Iso Specificity
III) Auto Specificity
IV) Organ Specificity and
V) Heterogenetic specificity.
Slide 20
ANTIGENIC SPECIFICITY
Basis of Ag specificity is stereo - chemical
Ag specificity is determined by a single chemical group
even by a single acid radical
The importance of position (ortho., meta and para) of
the antigenic determinant responsible for Ag specificity
Ag specificity is not absolute. Cross reaction, between
species may occur.
Slide 21
Species specificity:
Tissues of all individuals in a single species share a
common antigen. Cross reaction may occur to Ag
similarity
Tracing evolutionary relationships between species
Identification of species by blood and seminal stains
Slide 22
Iso specificity
Iso antigen are found in some but not all the members of
the species.
Eg: Blood group Ags, Rh antigens, Histocompatibility
antigens (HCA)
Importance: Transfusion, Iso-immunization, Homograft rejection
Disputed paternity
Anthropological study
Histocompatibility Ag; Cellular determinants specific for
each individual exist in the plasma membrane of the
cell.
Slide 23
Auto specificity
Self or auto antigens are normally not antigenic
Circumstances where self Ag behave foreign
- Sequestrated Ags: Lens protein, spermatozoa etc.
- Release into circulation (Infection, Irradiation, by
maneuver) etc.
Slide 24
Organ specificity
Organs such as brain kidney and lens protein of different
species share a common antigen. Such antigens are
characteristics of organ are known as organ specific Ags
which lead to organ specificity.
Eg : Neuroparalytic complication following neural Antirabies
(Sheep) vaccine.
Slide 25
Hetero specificity
The same or closely related Ags may occur in different
biological species, classes and kingdom. (Heterophile
antigens)
Eg: Forssmann Antigen: Lipid carbohydrate complex widely
distributed in human being animals, birds, plants and
bacteria.
Ags are used in serological tests unrelated to causative
agents (Weil-Felix for typhus fever, Paul-Bunnel in
infectious mononucleosis, Cold Agglutination test in Primary
atypical pneumonia, VDRL (Cardiolipin) for syphilis.
Slide 26
Superantigen

Super antigens are molecules that interact with antigen
presenting cells(APC) non-specifically without undergoing
endocytic processing.

Interaction of superantigens with MHC classII molecules of
APC and the VB domain of T lymphocyte receptor

This activates large number of lymphocytes as compared
to conventional antigen processing.

Massive cytokine expression and immunomodulation takes
place.Eg: Staphylococcal Toxic shock syndrome toxin,