What is the basic principle of vaccination? How do vaccines prevent microbial infections? Name the organism from which hepatitis $B$ vaccine is produced.

(N/A) The principle of vaccination is based on the property of 'memory' of the immune system.
In vaccination,a preparation of antigenic proteins of pathogens or inactivated/live but weakened pathogens is introduced into the body.
The antigens generate the primary immune response by producing antibodies. The vaccines also generate memory $B$-cells and $T$-cells.
When the vaccinated person is attacked by the same pathogens,the existing memory $B$-cells or $T$-cells recognize the antigen quickly and overwhelm the invaders with a massive production of lymphocytes and antibodies.
Hepatitis $B$ vaccine is produced from yeast.