Solution:
The principle of immunisation or vaccination is based on the property of memory of the immune system. In vaccination, a preparation of antigenic proteins of pathogen or inactivated/weakened pathogen (vaccine) are introduced into the body. The vaccines generate memory B cells and memory T cells that recognise the pathogen quickly on subsequent exposure to the pathogen and overwhelm the invaders with a massive production of antibodies. These antibodies would neutralise the pathogenic agents during actual infection.
