The goal of this observational study is to learn about if new biomarkers such as gut microbiota and molecular genetics melanoma features could predict clinical radiological and pathological response to neoadjuvant monotherapy with anti-PD1 agents in patients with resectable stage IIIB-D melanoma. The main questions it aims to answer are: * radiological and pathological response rate to three doses of antiPD1 agents; * do radiological and pathological responses correlate with gut microbiota and melanoma molecular genetics features Participants will receive three doses of aPD1 monotherapy as per center routine practice and will undergo regional lymphadenectomy. Before treatment initiation patients will be asked to bring faeces probes and fill out dietary questionnaire as well as just before the surgery. After sugery adjuvant therapy will be prescribed for 12 month and patients will be followed up according to institutional routine practice for 5 years.