The neutrophil-to-lymphocyte ratio (NLR), calculated as a simple ratio between the neutrophil and lymphocyte counts measured in peripheral blood, is a biomarker which conjugates two faces of the immune system: the innate immune response, mainly due to neutrophils, and adaptive immunity, supported by lymphocytes. Neutrophils are responsible for the first line of host immune response against invading pathogens, through different mechanisms, including chemotaxis, phagocytosis, release of reactive oxygen species (ROS), granular proteins and the production and liberation of cytokines.

Neutrophils also play an important regulatory role in adaptive immunity and are the main effector cells during the systemic inflammatory response (SIRS). As regulators of innate immunity, neutrophils recruit, activate and programme other immune cells, secreting an array of pro-inflammatory and immunomodulatory cytokines and chemokines capable of enhancing the recruitment and effector functions of other immune cells, such as dendritic cells (DCs), B cells, NK cells, CD4, CD8 and ?? T cells, as well as mesenchymal stem cells.

An isolated rise in neutrophil count, and, consequently, an elevated NLR, can be observed in several conditions: bacterial or fungal infection, acute stroke, myocardial infarction, atherosclerosis, severe trauma, cancer, post-surgery complications and any condition characterized by tissue damage that activates SIRS.

Several factors that could determine a “false” increase in NLR are age, exogenous steroid intake, endogenous sexual hormones, active haematological disorders, such as leukaemia, cytotoxic or granulocyte colony stimulating factor (G-CSF) chemotherapies, and HIV.