Final answer:
Active immunity occurs when one contracts a disease and the body produces memory cells, specifically memory lymphocytes (B or T cells), leading to a protective immune response upon subsequent exposures to the same pathogen.
Step-by-step explanation:
The type of immunity that occurs when one contracts a disease and produces memory cells is known as active immunity. This form of immunity is part of the adaptive immune system and is characterized by the formation of memory lymphocytes, such as B or T cells, in response to exposure to a specific pathogen. When a person is first exposed to a pathogen, a primary immune response occurs, and memory cells are generated. These cells are long-lived and can quickly become active upon re-exposure to the same pathogen, often preventing it from causing a serious infection or any symptoms of disease due to a rapid secondary adaptive immune response.
Vaccination is a method that artificially exposes the immune system to an antigen without causing disease, prompting the production of antibodies and memory cells in preparation for possible future infection by the actual pathogen. Thus, active immunity provides the biological basis for vaccination and the concept of immunological memory which protects us from getting diseases repeatedly from the same pathogen.