Low-power wide-area network technologies allow sending sensor information through long distances with low data rates. Long Range (LoRa) is one of these technologies, which is less susceptible to noise and interference. This technology operates in unlicensed bands, increasing the chances of having interfering devices in the communication link. The issue of LoRa jamming with external interference and the effect of such interference in radio range has not been addressed in the literature. Therefore, tests measured packet loss, received signal strength, and signal-to-noise ratio with one end node located kilometers away from a gateway. Data were measured with and without external interference, to establish system robustness. This article computes an empirical model of the immunity region, where the level of interference does not affect LoRa communication. Results show that LoRa devices required interference levels up to 14 dBm to have total packet losses. These results are relevant for implementing LoRa systems, motivating researches to perform site surveys of interference and to account for range reduction in network design.