**1. Neutrophils in the Central Nervous System (CNS)**

Neutrophils, crucial cells of innate immunity, are scarce in the central nervous system (CNS) under normal conditions. They are restricted from trafficking into the brain parenchyma and cerebrospinal fluid (CSF) by the presence of the brain–blood barrier (BBB). Tight junctions between brain endothelial cells ensure barrier integrity and high selectivity [1–3]. Yet, the infiltration of the CNS by neutrophils in various pathological conditions, e.g., infection, trauma, brain ischemia, neurodegeneration or autoimmunity, is a well-known phenomenon. Different studies have shown that neutrophil products, i.e., free oxygen radicals and proteolytic enzymes including matrix metalloproteinase 9 (MMP-9), play an important role in the pathogenesis of BBB damage [4,5]. It was recently observed that accumulating granulocytes may also release extracellular web-like structures composed of DNA and proteins called neutrophil extracellular traps (NETs), which damage the BBB and account for subsequent injury of surrounding neurons and other cells of the brain [1,6].
