Immunization

Immunization, or immunisation, is the process by which an individual's immune system becomes fortified against an infectious agent. When this system is exposed to molecules that are foreign to the body, called non-self, it will orchestrate an immune response, and it will also develop the ability to quickly respond to a subsequent encounter because of immunological memory. This is a function of the adaptive immune system.

Therefore, by exposing a human, or an animal, to an immunogen in a controlled way, its body can learn to protect itself: this is called active immunization.
The most important elements of the immune system that are improved by immunization are the T cells, B cells, and the antibodies B cells produce.
Memory B cells and memory T cells are responsible for a swift response to a second encounter with a foreign molecule.

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Source: Wikipedia · Image: Wikimedia Commons · Public domain