Chemotherapy is a direct form of attack on rapidly-dividing cancer cells, but this can affect other rapidly dividing cells including normal cells. When patients respond, the treatment’s effects happen immediately. These direct effects of chemotherapy, however, last only as long as treatment continues.

Immunotherapy treats the patient’s immune system, activating a stronger immune response or teaching the immune system how to recognize and destroy cancer cells. Immunotherapy may take more time to have an effect, but those effects can persist long after treatment ceases.

Immunotherapy treatments may take longer to produce detectable signs of tumor shrinkage compared to traditional therapies. Sometimes tumors may appear to grow on scans before getting smaller, but this apparent swelling may be caused by immune cells infiltrating and attacking the cancer. Many patients who experience this phenomenon, known as pseudoprogression, often report feeling better overall.

In certain cancer types, immune-related side effects may be linked with treatment success—specifically, melanoma patients who develop vitiligo (blotched loss of skin color)—but for the vast majority of patients, no definitive link has been established between side effects and immunotherapy’s effectiveness.