A chimeric antigen receptor (CAR) is a genetically engineered protein used in immunotherapy to help the immune system recognize and attack cancer cells. CARs are designed to bind to specific proteins, known as antigens, found on the surface of cancer cells. By modifying a patient's own T cells to express CARs, these immune cells can more effectively target and destroy tumors. The process of creating CARs involves extracting T cells from a patient, modifying them in the laboratory to express the CAR, and then infusing them back into the patient. This approach has shown promise in treating certain types of blood cancers, such as leukemia and lymphoma, by enhancing the body's natural ability to fight cancer.