Herceptin (trastuzumab) is a humanized monoclonal antibody that blocks the HER2 protein (human epidermal growth factor receptor 2) on the surface of cancer cells. As a monoclonal antibody, it substitutes antibodies to help the immune system. Mayo clinic dives into additional details here.
Herceptin is used to treat breast cancer that has spread to other body parts or has returned after surgery, targeted therapy, or chemotherapy. Blocking HER2 receptor activity allows Herceptin to prevent cancer cells from multiplying and growing.
It comes in two forms. First, Herceptin, which is administered via intravenous infusion. Second, Herceptin Hylecta, which is administered via subcutaneous injection. Both Herceptin and Herceptin Hylecta contain trastuzumab. However, Herceptin Hylecta also includes hyaluronidase-oysk.