The enigmatic neutrophil: what we do not know.

The neutrophil appears to be undergoing a renaissance of sorts. While it was for many years thought to be a killing machine brought into tissues to eradicate pathogens, it is now being implicated in many other processes, ranging from acute injury and repair, chronic inflammatory processes, cancer and auto-immunity. Not only is it an effector of the innate immune response, it appears to also potentially contribute to adaptive immunity, implicated in either contributing to the development of specific adaptive immune responses or perhaps even instructing and directing certain adaptive immune responses. With this renewed interest in the neutrophil and its numerous new functions, it is worth examining not what we know but rather what we do not know and what still needs to be more thoroughly examined. In this review, consideration is given to such topics as neutrophil subtypes, neutrophil differentiation, neutrophil as a director of immunity, neutrophil residency and ultimately death of the neutrophil.