Neutrophil extracellular trap, coloured scanning electron micrograph (SEM). Helicobacter bacteria (blue) caught in an immune system response known as a neutrophil extracellular trap (NET, green). These are networks of extracellular fibres, composed of DNA from neutrophils, which trap and bind pathogens such as bacteria. NETs allow neutrophils to kill extracellular pathogens while keeping damage to the host cells to a minimum.