Neutrophils are fundamentally part of the innate immune system (although there is cross-over with the adaptive immune system), and they play a fundamental role in the acute inflammatory response. The major function of neutrophils is to act as phagocytes - they are capable of ingesting and then destroying pathogens and debris. They are particularly effective in dealing with most types of bacteria and protozoa, and are also crucial in getting rid of many fungi. They seldom have a large role to play with viral infections or parasitic infestations, however. There's an amazing video of neutrophils phagocytosing several fungi here.
The first line of defence for an invaded tissue is usually the tissue's resident macrophages. However, they are small in number, and so within minutes to hours, a wave of neutrophils enters the infected area, drawn there from the blood stream by chemotaxis. Neutrophils, compared with macrophages, are much shorter-lived, and they are only capable of phagocytosing a handful of pathogens before becoming deactivated and dying. However, they have the virtue of being quick to produce and quick to get to the affected area. After a few hours to a day, a second wave of macrophages gets to the affected area and takes over the major phagocytic job.
Neutrophils are stocked with a whole host of nasty chemicals that aim to destroy any ingested pathogen. These chemicals are stored in vesicles and give neutrophils a granulated appearance (accounting for their being classified as one of the granulocytes). The substances in these 'granules' aim to obliterate via two main methods:
- Oxygen dependent - this category includes compounds like hydrogen peroxide, superoxide anion, and hypochlorous acid (a.k.a. chlorine bleach!)
- Oxgyen independent - this category includes a diverse range of antimicrobial substances, like lactoferrin (ties up any free iron so that the microbes can't get it), defensins (damage microbial membranes), bacterial permeability increasing protein and lysozyme (degrades bacterial peptidoglycan)
It obviously isn't important to know the names of most of these substances - I just listed a few to give you some idea.