Immunity refers to the body's ability to resist infections and diseases. There are two main types: innate immunity and adaptive immunity. Innate immunity is the body's first line of defense, providing immediate but non-specific protection against pathogens through barriers like skin and mucous membranes, as well as immune cells that respond quickly to invaders. Adaptive immunity, on the other hand, develops over time and is specific to particular pathogens. It involves the activation of B cells and T cells, which remember past infections and can mount a stronger response upon re-exposure. This type of immunity is the basis for how vaccines work, training the immune system to recognize and fight specific threats.