This lesson is designed to serve as an introduction to the immune system. It can stand alone or it can lead into further studies of the immune system. The primary focus of this inquiry-based lesson is antigen-antibody specificity. After focusing the learners' attention on allergic reactions to foods, two hands-on experiments allow learners to explore the specific reaction between an antigen and the antibody that recognizes it. Learners carry out an exploratory experiment leading to the concept of specificity. A second experiment allows learners to apply the techniques and concepts learned in the first activity and subsequent discussion to solve a mystery. This lesson can be used in association with topics such as allergies, food safety, or antigen-antibody precipitation. An entire packet complete with preparation instructions and student sheets can be obtained from the St. Louis Mathematics and Science Education Center (address listed in resource).