Pembrolizumab, sold under the brand name Keytruda, is a humanized antibody used in cancer immunotherapy that treats melanoma, lung cancer, head and neck cancer, Hodgkin lymphoma, stomach cancer, cervical cancer, and certain types of breast cancer. It is administered by slow intravenous injection. It is an IgG4 isotype antibody that blocks a protective mechanism of cancer cells, allowing the immune system to destroy them. It targets the programmed cell death protein 1 (PD-1) receptor of lymphocytes by blocking its interaction with PD-L1, resulting in the activation of T-cell-mediated immune responses against tumor cells.