This document discusses antigens and haptens. It defines antigens as macromolecules that elicit an immune response through antibody formation. It classifies antigens as exogenous or endogenous, and further divides endogenous antigens. It distinguishes immunogenicity from antigenicity and defines haptens as low molecular weight compounds that are antigenic but not immunogenic unless coupled to a carrier protein. It provides examples of haptens and describes tests used to detect antigens and haptens.

1. Antigens & Haptens
Name - Kajal Sharma
Course – B. Pharm (8th sem)
College – K.R. Mangalam University

2. Antigens
• Antigen is defined as ‘a macromolecular entity that
essentially elicits an immune response via the formation
of specific antibodies in the body of the host’.
• They are the substances that are capable of inducing
specific immune response.
• They can be recognized by the surface antibody(B cells)
or by the T cell receptors when associated with MHC
(Major histocompatibility complex) molecule.
• They can bind specifically with the antibody or T- cell
receptor.
MOST IMPORTANTLY
All immunogen are antigen but all antigen are not immunogen
(An immunogen is any antigen that is capable of inducing humoral and/or cell-mediated immune response)

3. Classification of Antigens
• EXOGENOUS ANTIGENS : Exogenous antigens are
those antigens which enter into the host body from their
surroundings or external environments. These are
basically of pollutants, microorganisms, pollens, drugs
etc.
• ENDOGENOUS ANTIGENS : They are again classified
onto-
• Xenogenic or Heterogenic antigens
• Allogenic or Idiotypic antigens
• Autologous antigens.

4. • XENOGENIC ANTIGENS : Foreign items which are related
with tissue transplantation and serology.
• ALLOGENIC ANTIGENS : Foreign items which enter when
an individual receives a blood transfusion or undergoes
transplantation operation.
• AUTOLOGOUS ANTIGENS : This group of antigens is very
rare and unnatural. In normal condition, self-components are
non-immunogenic in nature, but in an abnormal condition self-
body components are started to be considered as non-self or
antigenic component.

5. Immunogenicity v/s Antigenicity
• IMMUNOGENICITY : Ability to induce a humoral or
cell mediated immune response.
• ANTIGENICITY : Ability to combine specifically with
the final products of humoral or cell mediated immune
response.
• All molecules possessing the property of immunogenicity
also possess the property of antigenicity.
• Some molecules that possess the property of antigenicity
but lack immunogenicity is known as haptens.

6. Factors that influence
Immunogenicity
1. Contribution of the Immunogen
• Foreignness
• Molecular size
• Chemical Composition
• Physical Form
• Degradability
2. Contribution of the biological system
• Genetic factors
• Age
3. Method of administration
• Dose
• Route
• Adjuvants

7. Haptens
• Haptens are low molecular weight compound or molecules.
• Small organic molecules that are antigenic but not
immunogenic.
• They can bind to antibodies by themselves but they are not
recognised by the immune cells.
• And hence they fail to induce immune response, as a result no
antibodies to act against hapten.
• WHY HAPTENS ARE NOT IMMUNOGEN ?
• Haptens cannot bind to MHC/MHC-II (Major
histocompatibility complex). Hence cannot bind to TCR (T-
cell receptor).
• Results in no immune response

8. • Haptens can be immunogenic if they are covalently coupled
with a carrier protein.
• The carrier protein must be a larger protein having a molecular
weight of above 10,000 Da.
• This hapten-carrier conjugate is recognized by immune cells
and processed and presented to T-cell by MHC-II.
• Hence antibodies are generated. When hapten carrier conjugate
is introduced in body antibodies are generated against hapten,
carrier protein or conjugate.

9. Examples of Haptens
• Urushiol from the plant poison ivy.
• Hydralazine (generates autoimmune disease, Lupus
erythematosus).
• Nickel ion (ACD-allergic contact dermatitis).
• Halothane (cause hepatitis).
• Penicillin ( hemolytic anemia).

10. Tests
Test for Antigen :
i. Direct ELISA
ii. Direct Immunofluorescence
iii. RIA ( Radioimmunoassay technique)
iv. Neutralization test
v. CFT (Complement fixation test)
vi. Immunohistochemistry
vii. rapid antigen test (RAT)
Test for Hapten :
i. Patch Test procedure

11. References
1. Sambamurthy K, Kar Ashutosh. Pharmaceutical
Biotechnology. New Delhi: New Age International
Publishers; 1st edition 2006. p 25-27.
2. Punt J, Stanford S, Jones P, Owen A. Judith. Kuby
Immunology. United States: W.H Freeman & Company;
8th edition 2018. p 23-35.