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

This phenomenon is called 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 specific immunoglobulins against that antigen multiply and in days synthesize their antibodies this is called as primary response. Few of these specific B lymphocytes remain in the circulation for a long time, sometimes during the entire life of the individual, and they become the memory cells of the immune system when the body is exposed in the future to the same antigen the production of antibodies will be more intense and faster since the immune system is already prepared to react against that antigen this is called as the secondary response.