Since current computer infrastructures are increasingly vulnerable to malicious activities, intrusion detection is necessary but unfortunately not sufficient. We need to design effective response techniques to circumvent intrusions when they are detected. Our approach is based on a library that implements different types of countermeasures. The idea is to design a decision support tool to help the administrator to choose, in this library, the appropriate countermeasure when a given intrusion occurs. For this purpose, we formally define the notion of anti-correlation which is used to determine the countermeasures that are effective to stop the intrusion. Finally, we present a platform of intrusion detection that implements the response mechanisms presented in this paper.