Herceptin is part of a group of medications called monoclonal antibodies. Monoclonal antibodies are used to treat a wide variety of conditions, including cancer. Herceptin is a special type of monoclonal antibody that is designed to bind to HER2, a protein on the outside of certain cancer cells. When Herceptin binds to HER2 receptor proteins, it has several effects. It may interfere with the cancer cells' ability to grow and multiply. By binding to the HER2 receptor, Herceptin may also serve as a signal to the immune system, which can help destroy the cancer cells.
Herceptin is not a form of chemotherapy. It is less "toxic" than chemotherapy, as it specifically targets the HER2 receptors. Because Herceptin is only used to treat cancers that have a high concentration of HER2 receptors, it targets the tumors, having less of an effect on healthy, noncancerous cells.
Before Herceptin can be prescribed for a particular patient, certain tests must be performed to make sure that the tumor "overexpresses" HER2. When a cell makes a certain receptor or protein, it is said to "express" that receptor or protein. Tumor cells that overexpress HER2 have a high concentration of the HER2 receptor. Not all cancers overexpress HER2, and not all people with breast or stomach cancer will benefit from Herceptin.