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

Explanation

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