What is a vaccine ? How do they act to provide long term immunity to an individual who is vaccinated ?

A vaccine is a biological preparation that provides active acquired immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism (pathogen) and is often made from weakened or killed forms of the microbe, its toxins, or one of its surface proteins.

When vaccination is done, the antibodies produced in the body against these antigens neutralise the pathogenic agents during actual infection. The vaccines also generate memory – B and T-cells that recognise the pathogen quickly on subsequent exposure and overwhelm the invaders with a massive production of antibodies, thereby providing long-term effects.