Immunotherapy is a cancer treatment that helps your immune system fight cancer. It is a type of biological therapy. Biological therapy uses substances that are made from living organisms, or versions of these substances that are made in a lab.
Doctors don't yet use immunotherapy as often as other cancer treatments, such as surgery, chemotherapy, and radiation therapy. But they do use immunotherapy for some types of cancer, and researchers are doing clinical trials to see whether it also works for other types.
When you have cancer, some of your cells begin to multiply without stopping. They spread into the surrounding tissues. One reason that the cancer cells can keep growing and spreading is that they are able to hide from your immune system. Some immunotherapies can "mark" your cancer cells. This makes it easier for your immune system to find and destroy the cells. It is a type of targeted therapy, a treatment that specifically targets cancer cells. Other types of immunotherapies work by boosting your immune system to work better against cancer.
You could get immunotherapy intravenously (by IV), in pills or capsules, or in a cream for your skin. For bladder cancer, they might place it directly into your bladder. You may have treatment every day, week, or month. Some immunotherapies are given in cycles. It depends on your type of cancer, how advanced it is, the type of immunotherapy you get, and how well it is working.
You may have side effects. The most common side effects are skin reactions at the needle site, if you get it by IV. Other side effects may include flu-like symptoms, or rarely, severe reactions.
NIH: National Cancer Institute