Volume 1 - Issue 4, November - December 2025

Breast cancer develops within a complex tumor microenvironment that includes not only neoplastic epithelial cells but also a diverse repertoire of immune and stromal cells, soluble mediators, and extracellular matrix. The immune compartment—comprising tumor infiltrating lymphocytes, tumor associated macrophages, myeloid derived suppressor cells, regulatory T cells, and other subsets—plays a decisive role in tumor initiation, progression, metastatic spread, and response to systemic therapy. Distinct breast cancer subtypes, particularly triple negative and HER2 positive tumors, display characteristic immune phenotypes that can be captured by histopathology, immunohistochemistry, and emerging spatial and single cell technologies. These immune patterns have important prognostic and predictive value, informing patient stratification for chemotherapy and immunotherapy, especially immune checkpoint blockade. This paper reviews the cellular and molecular components of the breast cancer immune microenvironment, emphasizes their spatial organization and functional crosstalk, summarizes pathological methods for their assessment, and discusses how these features are being leveraged to develop microenvironment targeted therapies and to refine personalized treatment strategies.