Antigen - Copy.pptx

DR.KRISHNA AGARWAL
ASSISTANT PROFESSOR
GIMSR
ANTIGEN

Universities Press
© Universities Press (India) Private Limited
ANTIGEN
 Substance - introduced parenterally
 Production of antibody
 Reacts specifically and in an observable manner
• Parenteral - outside GIT
• Specifically - particular immunocytes
• Some antigens do not induce antibody
 Sensitise specific lymphocytes

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ANTIGEN
 Antigen – Interact specifically with antibodies, B
cell receptors and / or T cell receptors
 Immunogen – Induces detectable immune
response (humoral/cellular). Results in either
production of antibodies or activation of T cells.

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Antigenicity
 Immunogenecity
 Immunological reactivity

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ANTIGENS - 2 types
Complete antigen:
 Induce antibody - specific and observable
reaction
Hapten
 Incapable of inducting antibody
but react specifically with antibodies
 Hapten - Immunogenic on combining with larger
molecule carrier

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HAPTEN
Complex
 Precipitate with specific antibody
 Multivalent
Simple
 Non-precipitating
 Monovalent

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EPITOPE
 Smallest unit of antigenicity
 Antigenic determinant / epitope
 Epitope - 4 or 5 amino acids or monosaccharides
 Specific chemical structure , electrical charge &
Steric configuration
 Sensitising immunocyte
 Reacts with complimentary antibody or TCR

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Structure of epitope

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EPITOPE
 Linear or sequential epitope - single linear
segment of primary sequence.
Epitopes: linearised

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EPITOPE
 Conformational epitope – epitopes formed by
being brought together on surface residues from
different sites on peptide chain
Epitopes: conformational

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EPITOPE
 T cells recognise sequential epitopes
 B cells recognise conformational epitopes
 Paratope – combining area on antibody
corresponding to epitope
 Antigenic mosaic - bacteria or virus
 Antigenic cross reaction

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DETERMINANTS OF ANTIGENICITY
Size:
 Large molecules - highly antigenic
 Low molecular weight - weakly antigen
Chemical nature
 Naturally occurring antigens
Proteins / polysaccharides
 Lipids/Nucleic acid-less antigenic

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Susceptibility to tissue enzymes:
 Antigens – degraded fragments
 Phagocytosis and intracellular enzymes – antigens,
immunogenic fragments
 Synthetic polypeptides (D-amino acids) - not
antigenic,
 L amino acids - antigenic

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Foreignness ( non self )
 Induce immune response
 Ehrlich - ‘Horror autotoxicus’
 Tolerance to self antigen - during
development
 Breakdown of mechanism - autoimmunity

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ANTIGEN SPECIFICITY
 Determined - single chemical grouping
Species specificity:
 All individual in a species
 Tracing evolutionary relationships - species
 Forensic applications
blood ,seminal stains, etc.

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Isospecificity:
• lso antigens found in some but not all members
eg :Human erythrocyte antigens- blood groups
Applications
• Blood transfusion
• Isoimmunisation - pregnancy
• Disputed paternity cases
• Anthropology
• HLA antigens

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Auto specificity: Self-antigens
Sequestrated antigens – not recognised as self-
antigens
Eg ; eye lens protein , sperm
Organ specificity:
Some organs – brain kidney, lens protein of
different species share same antigen
Eg . Neuroparalytic complications – rabies
vaccine

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HETEROPHILE SPECIFICITY
Closely related antigens - different biological
species, classes, kingdoms
Heterogenetic /h eterophile antigens:
 Frossman antigen
Heterophile reactions
 Weil- Felix reaction : Typhus fever
 Paul-Bunnel test : Infectious mononucleosis
 Cold agglutinin test- primary atypical pneumonia

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BIOLOGICAL CLASSES OF ANTIGEN
T cell dependent (TD) antigens
T cell independent (TI) antigens

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T cell dependent (TD) antigens
 Most natural proteins are T cell dependent
 Require preliminary processing
 Induce a full gamut of immunoglobulin types IgM,
IgG, IgA and IgE
 Show immunological memory
 Rapidly metabolised in the body

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T cell independent antigen
 Structurally simple - composed of limited number
of repeating epitopes
Eg : Pneumococcal capsular polysaccharide,
bacterial lipopolysaccharide
 Does not require preliminary processing
 Antibody response limited to IgM and IgG3
 Do not show immunological memory

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Differences :
T cell independent
 Antigens directly stimulate
antibody production by B
cells without apparent
participation of T cells.
 Structurally simple
 Eg: capsular
polysaccharides.
 Immune response is
critically dose dependent.
 Metabolise slowly .
 Immunological memory is
absent.
 Ab response is limited to Ig
M & Ig G3
T cell dependent
 Require T cell participation to
generate immune response.
 Structurally complex.
 Eg:erythrocytes , serum
proteins
 Immunogenic over a wide
dose range & do not cause
tolerance readily.
 Rapidly metabolise.
 Immunological memory is
present.
 Induce Ig M, Ig G, Ig A, Ig E.

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SUPERANTIGENS
 Superantigens are certain protein molecules
such as staphylococcal enterotoxins, that
activate a large number of T cells, irrespective of
their antigen specificity

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SUPERANTIGENS
 Superantigens bind outside the antibody binding
groove directly to the lateral aspect of the TCR β
chain
 Conventional antigen fragments bind to the άβ
heterodimer groove of the MHC molecule
through the V region of the TCR ά and β chains

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SUPERANTIGENS
Schematic diagram of a superantigen

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Consequences :
Shock : mediated by massive release of
cytokines.
Immunosupression -- due to uncoordinated
activation of immune system.
Autoimmunity – due to bypass of
autoreactive T & B cells.

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Superantigens
• Staphylococcal superantigens :
Staphylococcus enterotoxin A – E
Staphylococcus enterotoxin G – P
Toxic shock syndrome toxin 1.
• Streptococcal superantigens -
Str pyogenes exotoxin A,B,C,E & G1
Streptococcal mitogen exotoxin Z
(most potent of superantigens)

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• Pseudomonas - Exotoxin A
• Yersinia pseudotuberculosis -
Yersinia pseudotuberculosis mitogen (YPM)

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Disease associated with superantigen
production:
• Toxic shock syndrome
• Scarlet fever
• Kawasaki disease
• Eczema
• Psoriasis
• Rheumatic arthritis

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DETERMINANTS RECOGNISED BY INNATE IMMUNE
SYSTEM
 Innate immune system recognise unique
molecular patterns - pattern recognition
 Pathogen associated molecular patterns
(PAMPs) - broad molecular patterns
 Pattern recognition receptors (PRRs) –receptors
for PAMPs

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PATTERN RECOGNITION RECEPTORS
TYPES
 Toll-like receptors (TLRs) – transmembrane
receptors present on macrophages and dendritic
cells
 Scavenger receptors – include CD 36, CD 68
and SRB-1
 Mannose receptors - receptors on the surface
of phagocytes bind mannose-rich glycans found
in microbial glycoproteins.

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THANK YOU

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