Chemotherapy is given before CAR-T cell therapy primarily to prepare the body for the treatment and enhance its effectiveness. This preparative chemotherapy, also known as lymphodepletion, reduces the number of existing immune cells, creating space for the infused CAR-T cells to expand and function more effectively. It also lowers levels of certain immune-suppressing cells that could interfere with the activity of CAR-T cells. By doing so, chemotherapy helps create an environment where the modified T cells can thrive, multiply, and better attack the cancer cells. Additionally, lymphodepletion may reduce the tumor burden slightly, which can help in managing side effects such as cytokine release syndrome—a common reaction to CAR-T therapy. The type and intensity of chemotherapy used vary depending on the patient’s condition and the specific CAR-T therapy protocol. Overall, chemotherapy before CAR-T therapy is a crucial step to improve the therapy’s success and durability.