Keytruda is a brand name for the drug pembrolizumab, which is a type of immunotherapy used to treat various types of cancer. According to the official Keytruda website [2], this drug works by blocking a specific protein called PD-1 on the surface of T cells, which are a type of immune cell. PD-1 normally acts as a "brake" on T cells, preventing them from attacking healthy cells in the body. However, some cancer cells can also produce a protein called PD-L1, which can bind to PD-1 and "trick" T cells into ignoring the cancer cells.

By blocking PD-1, Keytruda allows T cells to recognize and attack cancer cells more effectively. Specifically, Keytruda helps to "wake up" T cells that are already present in the body, but may be inactive or suppressed by the cancer [2]. This can help to shrink tumors and slow the progression of cancer.

It's worth noting that Keytruda is not effective for all types of cancer, and may not work for all patients. According to the National Cancer Institute [3], Keytruda is approved for use in several types of cancer, including melanoma, non-small cell lung cancer, head and neck squamous cell carcinoma, classical Hodgkin lymphoma, urothelial carcinoma, and others. However, the effectiveness of Keytruda may depend on factors such as the patient's specific type of cancer, the stage of the cancer, and other individual factors.

Sources:
[2] https://www.keytruda.com/how-does-keytruda-work/
[3] https://www.cancer.gov/publications/dictionaries/cancer-terms/def/keytruda