In medicine, the immune system’s way of protecting the body against an infectious disease. The three types of immunity are innate, active, and passive. Innate immunity includes barriers, such as skin and mucous membranes, that keep harmful substances from entering the body. It is the first response of the body’s immune system to a foreign substance. Active immunity occurs in response to being infected with or vaccinated against a microorganism. The body makes an immune response, which can prevent future infection with the microorganism. Active immunity can last a person’s entire life. Passive immunity occurs when a person receives antibodies to a disease rather than making them through his or her own immune system. Passive immunity provides immediate protection but only lasts a few weeks or months.