Pembrolizumab is primarily used in the treatment of various cancers by enhancing the body's immune response against tumor cells. It is particularly effective in treating melanoma, non-small cell lung cancer, and head and neck squamous cell carcinoma. The drug works by blocking the PD-1 pathway, which cancer cells often exploit to evade immune detection. This allows T-cells to recognize and attack cancer cells more effectively. Additionally, pembrolizumab is used in the treatment of Hodgkin lymphoma and certain types of gastric and bladder cancers. Its application has shown significant improvements in patient survival rates and is often used in combination with other therapies for enhanced efficacy.