Cellular immunity can be changed after surgery and anesthesia. Tissue injury by surgical procedures significantly reduces the immune function in proportion to the degree of tissue injury. In addition, the use of opioids as analgesics during the perioperative period depress cellular immunity, in particular the activity of the natural killer (NK) cells. This perioperative immunosuppression may cause deleterious consequences on postoperative outcome in cancer patients. The aim of this study is to confirm the presence or extent of the change of NK cell activity between before and after head and neck surgery. Additionally, investigators will identify the clinical or genetic factors related to the change of NK cell activity.

Inclusion Criteria:

1) The patients undergoing head and neck surgery including tumor removal and reconstruction with flap

Exclusion Criteria:

1. patients refusal
2. patients not able to read, or understand the consent form
3. History of Active viral or bacterial infection
4. History of use of immunosuppressive drugs within the last six months
5. History of chemotherapy
6. History of autoimmune disorders
7. Known HIV, Hepatitis B, or Hepatitis C infection
8. Female who is pregnant, nursing
9. ethnicity, other than asian

Patients undergoing head and neck surgery

no plan to share IPD