Cancer cells grow in an uncontrolled fashion. Herceptin works on the surface of the cancer cell by blocking the chemical signals that can stimulate this uncontrolled growth.
Genes are like instruction manuals that tell each cell of our body how to grow, what kind of cell to become, and how to behave. Genes do this by ordering the cell to make special proteins that cause a certain activity like cell growth, rest, or repair.
Some cancer cells have abnormalities in genes that tell the cell how much and how fast to grow. Sometimes the cancer cells have too many copies of these genes with abnormalities. When there are too many copies of these genes, doctors refer to it as "overexpression." With some forms of gene overexpression, cancer cells will make too many of the proteins that control cell growth and division, causing the cancer to grow and spread.
Some breast cancer cells make (overexpress) too many copies of a particular gene known as HER2. The HER2 gene makes a protein known as a HER2 receptor. HER2 receptors are like ears, or antennae, on the surface of all cells. These HER2 receptors receive signals that stimulate the cell to grow and multiply.
But breast cancer cells with too many HER2 receptors can pick up too many growth signals and so start growing and multiplying too much and too fast. Breast cancer cells that overexpress the HER2 gene are said to be HER2-positive.
Herceptin works by attaching itself to the HER2 receptors on the surface of breast cancer cells and blocking them from receiving growth signals. By blocking the signals, Herceptin can slow or stop the growth of the breast cancer. Herceptin is an example of an immune targeted therapy. In addition to blocking HER2 receptors, Herceptin can also help fight breast cancer by alerting the immune system to destroy cancer cells onto which it is attached.