What is vaccine ? What are its types ?

Explanation

A vaccine is a biological preparation that provides active acquired immunity to a particular disease. It resembles a disease causing microorganisms. The weakend or attenulated or killed forms ot microbes, their toxins or one of their surface protein constitute vaccine.
When a bacteria or viruses invade our body, vaccines defend it. Small amount of vaccine is enough to allow the immune system to recognize the pathogen without actually experiencing the disease. Booster vaccine need to be given more than once to make the immune system to overcome to future infection.
The types of vaccines are first generation vaccine, second and third generation vaccines.
(i) First generation vaccine : These vaccines are classified into live attenuated vaccine, killed vaccine and toxoids. The weakend, aged less virulent form of the virus are included under attenulated live vaccines. Eg. Measles, mumps and rubella (MMA) vaccine. The killed (inactivated) vaccines are killed pathogens by heat and other methods. Eg : Salk.s polio vaccine. A toxin or a chemical secreted by the bacteria or a virus can act as vaccine and such type of vaccine is called toxoid vaccine. Eg : DPT vaccine (Diphtheria Pertussis and Tetanus).
(ii) Second generation vaccine : The pure surface antigen of the pathogen acts as vaccine. Eg : Hepatitis B vaccine.
(iii) Third generation vaccine : They are synthetic vaccines, which contain purest and highest potential vaccines.