2. Antigen is a molecule that can bind to an
antibody, B cell receptor or T cell receptor
A stimulus that produces a humoral or cell-
mediated immune response
All immunogens are antigens but not all antigens are immunogens
Some very small molecules called haptens can bind to Ab’s or TCR’s but they cannot
initiate an immune response…
3. Haptens
• The term Hapten was given by the
immunologist Karl Landsteiner, who studied
them in early 20th century.
• It came from a Greek word Haptein meaning
to fasten.
4. Haptens
Haptens are low molecular weight
molecules
Small organic molecule that are antigenic
but not immunogenic
Which means that they can bind to
immune cells but fail to induce Humoral
or cell mediated immune response.
Hence no antibodies are raised against
them
5. Why Haptens are not
Immunogenic?
Cannot activate
Helper T cells
Failure to activate
Helper T cells is due
to their inability to
bind to MHC
proteins
Haptens are
univalent hence
cannot activateB
cells by themselves
6. • Haptens can induce immune response when covalently
bound to a large protein called a ‘carrier protein’ forming
Immunogenic Hapten-Carrier Conjugate.
• Carrier protein molecule are antigenic as well as
immunogenic.
A peptide of the carrier protein is presented in association with class II MHC protein
to the helper T cells. The activated helper T cells then produce interleukins, which
stimulates the B cells
8. Animals immunized with such a Hapten conjugate produce antibodies specific for :
(1) the hapten determinant,
(2)unaltered epitopes on the carrier protein
(3) new epitopes formed by combined parts of both the hapten and carrier
9. • By itself, a hapten cannot function as an
immunogenic epitope.
• But when multiple molecules of a single
hapten are coupled to a carrier protein (or
nonimmunogenic homopolymer), the hapten
becomes accessible to the immune system
and can function as an immunogen.
Hapten
Carrier
molecule
Immunogen
10. Pioneering work of Karl Landsteiner
Hapten molecule
injected
Carrier Molecule
injected
Hapten-Carrier Conjugate
injected
Anti-serum
Anti-serum
Anti-serum
12. By observing which hapten modifications prevented or permitted cross-
reactions, Landsteiner was able to gain insight into the specificity of antigen
antibody interactions
Pioneering work of Karl Landsteiner
(contd…)
• Landsteiner immunized rabbits with a hapten-carrier conjugate and then
tested the reactivity of the rabbit’s immune sera with that hapten and
with closely related haptens coupled to a different carrier protein
• He was thus able to measure, specifically, the reaction of the antihapten
antibodies in the immune serum and not that of antibodies to the original
carrier epitopes
• Landsteiner tested whether an antihapten antibody could bind to other
haptens having a slightly different chemical structure. Such reactions are
called a cross-reaction
• Antibodies developed against aminobenzene or its derivatives react only
with the original hapten and do not cross react with any other derivative.
13. Examples of Haptens
Many biologically important substances which can
function as haptens are:-
• Drugs- such as Penicillin
• peptide hormones
• steroid hormones
Some important Haptens are:
• Dinitrophenol
• Aminobenzene
• o-aminobenzoic acid
• m-aminobenzoic acid
• P-aminobenzoic acid
14. • The formation of drug-protein conjugates in the
body can produce drug allergies that may be life-
threatening.
Haptens in Real life...
• Immunologist’s favourite, as it can be used as-
DIAGNOSTIC TOOL
Enzyme-multiplies Immunoassay Technique
(EMIT)
15. Allergy to Drugs
Other drugs which may elicit allergy streptomycin,
aspirin, the so-called “sulfa-drugs” such as the
sulfonamides, some anesthetics (e.g., succinyl choline),
and some opiates.
