Antigen-driven bystander effect by Th2 pretreatment enhanced epicutaneously induced Th2 immune response to a new protein antigen. Groups of mice (four per group) were non-pretreated (PBS, no), or intraperitoneally (ip) pretreated with BSA and alum adjuvant on day -21. They were all epicutaneously sensitized with BSA and OVA (no, ip) or PBS (PBS) on day 1–5. On day 11, draining LNs were obtained (A). On day 22, spleens were taken (B). In vitro reactivation culture with OVA was performed. Cytokine contents of 48 h-supernatants were measured by ELISA. Results were shown as mean ± SEM. Four independent experiments were performed with similar results. One representative results are shown.