Antigens & haptens

An antigen is either a cell or molecule which will bind with pre exiting antibody but will not definitely cause induction of antibody production. Antigen may also be defined as ‘a macro molecular entity that essentially elicits an immune response via the formation of specific antibodies in the body of the host’. And Hapten may also be defined ‘as a substance that normally does not act as an antigen or stimulate an immune response but that can be combined with an antigen and, at a later time, initiate a specific antibody response on its own’.

1. Antigens & Haptens Name - Kajal Sharma Course – B. Pharm (8 th 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. T ests Test for Antigen : Direct ELISA Direct Immunofluorescence RIA ( Radioimmunoassay technique) Neutralization test CFT (Complement fixation test) Immunohistochemistry rapid antigen test (RAT) Test for Hapten : Patch Test procedure

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