Why do doctors always talk about lymph nodes in breast cancer? Lymph nodes are small glands that are part of your immune system. They help filter out harmful substances, including cancer cells. In breast cancer, we pay close attention to the lymph nodes in the armpit. Why? Because they’re often the first place breast cancer cells may travel to. If cancer is found in the lymph nodes, it doesn’t automatically mean it has spread everywhere. But it does help us understand the stage and decide on tre