Lymph nodes are small structures that filter substances in a fluid called lymph and help fight infection. In breast cancer, these nodes can become pathways for the cancer to spread. The axillary lymph nodes, located in the armpit, are most commonly affected, receiving about 85% of the breast's lymphatic drainage. Tumor size and location influence the likelihood of lymph node involvement, with larger and laterally located tumors being more likely to spread.

Besides axillary nodes, other lymph nodes like internal mammary and supraclavicular nodes can also be involved. Internal mammary nodes are more likely to be affected if the tumor is located medially in the breast. Supraclavicular nodes, found near the collarbone, are often involved when there's extensive spread in the axillary nodes.

When breast cancer spreads to lymph nodes, it can change the course of treatment. Treatments may include surgery, radiation, or chemotherapy, and the decision is based on the extent of lymph node involvement. For instance, if only a few nodes are affected, less extensive surgery might be possible, while more nodes may require comprehensive therapy.

Understanding lymph node involvement in breast cancer is crucial for tailoring the best treatment plan.