An immunization system including: an immunization client apparatus which determines whether a target code is a malicious code by performing an immunization operation with respect to a first immunization signature and a code signature that is extracted from the target code and reports the result of the determination to an immunization server; and the immunization server which diagnoses whether the target code is the malicious code, updates a second immunization signature based on the reported result of the determination, and transmits to the immunization client apparatus an update message about the updated second immunization signature, wherein the immunization client apparatus updates the first immunization signature based on the received update message is provided.