The construction of reliable reactive systems is considered to be one of the most challenging goals in the fields of software and system engineering. The definition of a reactive system suits biological systems at different levels, ranging from gene networks, developing embryos and the immune system. We report here the application of a tool developed for constructing computerized systems to the modeling and analysis of a biological system, the immune system. We use the language of statecharts within the framework of object-oriented modeling. The results described here indicate that this modeling strategy can contribute to the transition of biology from the phase of analysis to the phase of synthesis.