Active immunity arises when the body's immune system responds to direct exposure to pathogens or vaccination, resulting in production of antibodies and formation of memory cells for long-term protection. Conversely, passive immunity, obtained either naturally or artificially through transfer of pre-formed antibodies, offers immediate but temporary protection without the formation of memory cells.

The immune system defends the body against foreign entities through two mains systems: innate and adaptive immunity. Innate immunity offers immediate, non-specific responses to a broad range of pathogens, utilizing physical barriers and cells such as macrophages. Adaptive immunity, however, provides a more targeted defense, retaining memory of past encounters to enhance response upon re-exposure.