Abstract
<jats:p>The neutrophil-to-lymphocyte ratio (NLR) is a potential biomarker for the assessment of systemic inflammation and immune response. It is a simple, easily obtainable, and inexpensive marker, calculated from the ratio of neutrophil and lymphocyte counts in the peripheral blood system. NLR reflects two components of the immune system: innate immunity governed by neutrophils and adaptive immunity mediated by lymphocytes. The role of inflammation in cancer development and progression has been a topic of discussion for a long time. This chapter outlines the importance of biomarkers like NLR in systemic immune balance and explains how neutrophils and lymphocytes play an important role in the pathogenesis, progression, and biology of various cancers. The clinical role of NLR has been studied and reviewed in many solid cancers, with special emphasis on breast cancer. Numerous studies and meta-analyses have shown that raised NLR levels correspond to aggressive tumor behavior, poor prognosis, and increased probability of metastasis. In breast cancer, NLR has been evaluated in terms of clinicopathological parameters, molecular subtypes, and the Ki-67 proliferation index. Despite its advantages, there are various drawbacks and limitations of NLR in clinical settings, such as its variability in cut-off values and the influence of nonmalignant conditions that can alter its value and results. Future research, with the integration of other clinicopathological parameters and inflammatory biomarkers, may help in achieving standardized thresholds and strengthening its clinical application. To summarize, NLR is a potential biomarker of systemic inflammation and immune response associated with tumor progression in many solid tumors, particularly breast cancer.</jats:p>