– 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 the memory $B$-cells and $T$-cells.

When the vaccinated person is attacked by the same pathogens, the existing memory $B$-cells or $\mathrm{T}$. cells recognise the antigen quickly and overwhelm the invaders with massive production of lymphocytes and antibodies.

Hepatitis $B$ vaccine is produced from yeast