How can an organism that once underwent contact with an antigen be immunized against future infections by the same agent?

This phenomenon is known as immune memory. When an antigen makes contact for the first time with cells of the humoral immune system, B lymphocytes that are producers of exact immunoglobulins against that antigen multiply and in days synthesize their antibodies. This is known as primary response. Some of these exact B lymphocytes remain in the circulation for a long time, sometimes during the whole life of the individual, and they become the memory cells of the immune system. When the body is exposed in the future to the similar antigen the production of antibodies will be faster and more intense as the immune system is already prepared to react against that antigen. This is known as the secondary response.