Immunotherapy has been shown to be effective in treating melanoma, even at an early stage of disease. Patients with Stage II melanoma are now able to receive a class of medicines known as anti-PD-1 therapy, that boost the immune system’s fight against cancer following surgical removal of the tumor. Because most patients are cured after surgery alone, the challenge is to determine who really needs immunotherapy versus those who might get medicine they do not need and risk complications from the medicine. We seek to study the immune cells in the sentinel lymph node and in the blood to test whether these immune cells can serve as cellular “beacons” to indicate the presence of otherwise undetected residual tumor, which would help us identify who would benefit the most from immunotherapy. We will also determine if we can link immune cells in the blood to the cellular “beacons” in the sentinel lymph node and thus be able to monitor the effectiveness of immunotherapy. Together, these data will help us determine who is best served by immunotherapy, potentially opening the door to opportunities for better survival and fewer complications.