In this work, we describe an application to automatically generate chord progressions with an artificial immune system (AIS). We propose a penalty function that encodes rules about chord construction as vertical constraints and chord progression as horizontal constraints. Then we use a AIS to find chords that minimize the penalty function and propose the next chord in a sequence as a minimum-penalty chord given the sequence as input. The AIS used is opt-aiNet, an immunological algorithm capable of finding multiple optima in parallel upon convergence, resulting in several chords as candidates for the next in the sequence.