The scientific concepts of human immunology are inherently complicated and extremely difficult to understand. Hence, this study reports on the development of an educational game entitled "Humunology" and examines the impact of using "Humunology" for learning how the body's defense system works. A total of 132 middle school students participated in this study and a quasi-experimental approach with a two-group pretest/posttest research design was used. A knowledge assessment including 19 items was developed, and a questionnaire instrument with seven dimensions, which focuses mainly on perceptions toward the use of "Humunology," and the help-seeking behaviors of the students, was employed. The results show that students who learned by playing "Humunology" significantly outperformed those who learned by using web-based content on items that examined their understanding of procedural knowledge and higher level of cognitive process. Students in the experimental group also had a significantly higher level of satisfaction than their counterparts. In terms of predicting a student's learning achievement on the posttest, the three positive variables were the results of the pretest, perceived ease of use, peer learning and help-seeking behaviors. The only negative one was perceived playfulness. The implications and suggestions for further research derived from these findings are discussed.