Antigen is substance which when introduced parentally into the body stimulates the production of an antibody with which it reacts specifically and in an observable manner. CLASSIFICATION OF ANTIGENS Based on Immunogenicity Complete antigen : substances with both immunogenicity and immunoreactivity Incomplete antigen  Incomplete antigen ( hapten): substances only with immunoreactivity Hapten +carrier → complete antigen ( Immunogenicity : induction of immune response • Immunological Reactivity: specific reaction with antibodies or sensitized cells) 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. DEFINITION OF HAPTENS Small, non-biologic molecules that bind to immune cells receptors but cannot by themselves induce a specific immune response 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 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. DEFINITION OF HAPTENS Small, non-biologic molecules that bind to immune cells receptors but cannot by themselves induce a specific immune response 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

2. Jens Martensson 2
ANTIGEN
Antigen is substance which when introduced parentally into the body stimulates the
production of an antibody with which it reacts specifically and in an observable
manner.
CLASSIFICATION OF ANTIGENS
Based on Immunogenicity
Complete antigen : substances with both immunogenicity and
immunoreactivity Incomplete antigen
Incomplete antigen ( hapten): substances only with
immunoreactivity
Hapten +carrier → complete antigen
( Immunogenicity : induction of immune response
• Immunological Reactivity: specific reaction with antibodies or sensitized cells)

3. Jens Martensson 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.
DEFINITIONOF HAPTENS
Small, non-biologic molecules that bind to immune cells receptors but cannot by
themselves induce a specific immune response
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

4. Jens Martensson 4
Why Haptens are not Immunogenic?
1.Cannot activate Helper T cells
2. Failure to activate Helper T cells is due to their inability to
bind to MHC proteins
3. Haptens are univalent hence cannot activate B cells by
themselves
TYPES
Haptens may be Complex or Simple.
1.Complex Haptens Polyvalent ,Precipitate with Specific
antibodies.
2.Simple Haptens Univalent ,Non – precipitate with Specific
antibodies.

5. Jens Martensson 5
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

6. Jens Martensson 6

7. Jens Martensson 7

8. Jens Martensson 7
Pioneering work of Karl Landsteiner
• 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.

9. Jens Martensson 8
Similarities - Antigens & Haptens
1) Both are Antigenic.
2) Both are present on external cellular surfaces of microbial
pathogens and other agents
. 3) Both form a part of the defense mechanism system between
antigen and antibody.
4) Both have the ability to bind to the antibody via Weak linkages
such as Ionic interactions, Hydrogen bonding and Hydrophobic
interactions.

10. Jens Martensson 9
Differences - Antigens and Haptens
Antigens
An antigen is a foreign body, which has
the ability to trigger the host immune
system to produce an immune reaction by
binding to an antibody.
Antigen directly binds to the antibodies
produced and initiate an immune reaction.
Antigens are not conjugating with a carrier
molecule.
Antigens are used in in vitro techniques
such as ELISA and in Pharmacological
purpose
Antigen reactions are Antigenic and
Immunogenic.
Haptens
A Hapten is an incomplete antigen which
is not originally immunogenic
Hapten binds to an antibody but does not
have the ability to trigger the host immune
system to produce an immune reaction..
Haptens conjugate with carrier molecules.
Haptens are used in Antibiotics and
Anesthetics designing
Hapten reactions are only Immunogenic.

11. Jens Martensson 10
Examples of Haptens
1.Aniline (an Organic compound) and its derivatives (o-, m, p-
benzoic acid) (First researched Hapten).
2) Urushiol present in Toxicodendron radicans, Poison ivy plant.
3) Hydralzine, a blood pressure lowering drug.
4) Fluorescein, a fluorescent dye.
5) Biotin, a Vitamin – B7 (Formerly known as Vitamin – H or
Coenzyme – R).
6) Penicillin, an Antibiotic.
7) Digoxigenis, a steroid found in the flowers & leaves of the
plants Digitalis purpurea, Digitalis orientalis & Digitalis lanata.
8) 2,4 – Dinitrophenol, an Organic compound

12. Jens Martensson 12
Example of a Hapten is Penicillin, a small molecule and is not antigenic.
However, when it is combined with certain serum proteins of sensitive
individuals, the resulting molecule becomes immunogenic, activates
lymphocytes, and initiates a severe and sometimes fatal Allergic reaction (a
type of immune response).

13. Jens Martensson 13

14. Jens Martensson 14
Poison ivy plant.
Poison ivy, oak and sumac belong to a family of plants that produce one of the most
common allergic reactions in the United States. Experts estimate that up to 70
percent of the population is allergic to urushiol, the oil found in the sap of these
plants. The reaction, known as “urushiol-induced allergic contact dermatitis,” occurs
when urushiol attaches itself to the skin after a person’s direct or indirect exposure
to the oil.
Chemically, urushiol is harmless to humans.
• However, when contacted, it bonds to deep skin proteins.
• This initiates a T-cell mediated immune response towards the urushiol derivatives
which are bound up in the skin proteins.
• The result is an allergic reaction characterized by redness, swelling, and blisters.

15. Jens Martensson 15
Poison ivy plant.

16. Jens Martensson 16
Allergy reactions
Penicillin reaction Poison ivy
Poison oak

17. ANTIGENE