The immune system enables your body to distinguish its own healthy cells from abnormal or foreign cells and organisms. These foreign invaders include viruses, bacteria, and other disease-causing organisms, including cancer. Invading cells have special marker substances on their surfaces, known as antigens. When your immune system detects antigens, your body sends an array of immune cells to destroy or wall off the invading cells. (See A Guide to the Immune System for more information.)

Immunotherapy, the main focus of biological therapy, uses natural or manufactured substances to boost or restore the body's immune system so it can better fight against disease. Most commonly used as adjuvant therapy for melanoma following surgery, immunotherapy is also used to treat advanced and recurrent melanoma. Many clinical trials are underway worldwide to find effective therapies.

Established and experimental approaches in immunotherapy include:

• Interferons: naturally-produced proteins that help activate the immune system to fight disease
• Interleukins: naturally-produced proteins that help the immune system produce more infection- and cancer-fighting cells
• Combinations of immunotherapies, such as interferon and interleukin-2
• Biochemotherapy: the combination of immunotherapy and chemotherapy drugs to combat advanced or recurrent melanoma
• Cancer vaccines: proteins that mark tumor cells so they are recognized by the immune system. These proteins may enlist other components of the immune system, including T cells and B cells, to directly attack tumor cells or stimulate other immune cell function against tumor cells.
• Antibodies: proteins produced by B cells in response to an antigen. Each antibody is a customized receptor on the B cell surface and can bind to only a specific antigen on a tumor cell surface. This binding may stimulate certain immune cells to attack and destroy the tumor cells. Monoclonal antibodies are laboratory-produced antibodies used in the treatment of cancer, including melanoma. They may be used alone or used to deliver drugs or radioactive material directly to a tumor.