Assertion : Principle of vaccination is based on the property of 'memory' of the immune system.
Reason : In vaccination a preparation of antigenic protein of pathogens or inactivated/ weakened pathogen are introduced into the body.

Explanation

The principle of Vaccination and Immunization is based on the property of .memory. of the immune system.
In Vaccination, a preparation of antigenic proteins of pathogens or inactivated/weakened pathogen (vaccine) is introduced in the body.
The antibodies produced in the body against these antigens would neutralize the pathogenic agents during actual infection.
The Vaccine also generate memory B and T cells that recognize the pathogen quickly on subsequent exposure and overwhelm the invaders with massive production of antibodies.
If a person is infected with some deadly microbes to which quick immune response is required as in tetanus, we need to inject the performed antibodies or antitoxins (a preparation containing antibodies and toxin).
In case of snake bite the injection which is given to the patients, contain preformed antibodies against the snake venom. This time of immunization is called passive Immunisation.