Immunity, active: The production of antibodies against a specific agent by the immune system. Active immunity can be acquired in two ways, by contracting an infectious disease such as chickenpox or by receiving a vaccination such as against chickenpox. Active immunity is usually permanent. The individual is protected from the disease all their life.
Active immunity is in contrast to passive immunity which results from the transfer to an individual of antibodies produced by another individual. Passive immunity is limited and diminishes over time (usually a few weeks or months). For example, antibodies are passed from the mother to the baby before birth. These antibodies temporarily protect the baby for the first 4-6 months of life.