Immunotherapy has become a standard of care in treating high-risk, early-stage breast cancers, yet it has had limited success in shrinking tumors. New biomarkers that can improve outcomes for patients are urgently needed. Now, a study led by researchers at the Vanderbilt-Ingram Cancer Center has found that repeated blood sampling—essentially, a liquid biopsy—can assess and predict the evolving antitumor immune response to therapy.