Biotherapy: Treatment to stimulate or restore the ability of the immune (defense) system to fight infection and disease. Biological therapy is thus any form of treatment that uses the body's natural abilities that constitute the immune system to fight infection and disease or to protect the body from some of the side effects of treatment.

Biotherapy (also called biological therapy or immunotherapy) often employs substances called biological response modifiers (BRMs). The body normally produces low levels of BRMs in response to infection and disease. Large amounts of BRMs can be made in the laboratory to treat cancer, rheumatoid arthritis, and other diseases.

Forms of biological therapy include monoclonal antibodies, interferon, interleukin-2 (IL-2), and several types of colony-stimulating factors (CSF, GM-CSF, G-CSF). Interleukin-2 and interferon are BRMs being tested for the treatment of advanced malignant melanoma. Interferon is a BRM now in use to treat hepatitis C.

Biologic therapy to block the action of instruments of inflammation called tumor necrosis factor (TNF) is being explored to treat conditions such as Crohn's disease and rheumatoid arthritis. Etanercept (ENBREL) is a commercially available injectable TNF-blocking treatment for patients with severe rheumatoid arthritis.

The side effects of biological therapy depend on the type of treatment. Often, these treatments cause flu-like symptoms such as chills, fever, muscle aches, weakness, loss of appetite, nausea, vomiting, and diarrhea. Some patients develop a rash, and some bleed or bruise easily. In addition, interleukin therapy can cause swelling. Depending on how severe these problems are, patients may need to stay in the hospital during treatment. These side effects are usually short-term and they gradually go away after treatment stops.