Many existing MAC protocols for wireless sensor networks try to achieve simultaneously low latency and low power consumption. For many monitoring applications, however, there are two separated types of traffic. Most of the traffic is periodic, and is not subject to latency constraints: energy savings is the main objective for this traffic. A small proportion of the traffic has strict latency requirements, and the energy consumed for this priority traffic is not an issue. In this paper, we introduce a MAC protocol that can achieve this traffic differentiation in a WSN. We propose to combine the low power listening approach for periodic traffic with opportunistic encounters mechanisms for urgent traffic. We show that our approach provides a good ratio of consumed energy per delivered packet of the periodic traffic, while keeping a low loss rate for the priority traffic.