Neutrophils are highly specialized white blood cells abundant in blood, where they have a short half-life if they are not stimulated by specific chemokines/cytokines and recruited to a site of inflammation. The abundance of neutrophils in blood and, efficiency of their recruitment and antimicrobial action make these cells the first line defense of the immune system. By releasing toxic substances to the extracellular space, neutrophils contribute to Systemic Inflammatory Response Syndrome and cause tissue damage subjacent the acute respiratory distress syndrome and Multiple Organ Failure.

     The resting neutrophil proteome is elucidated by trypsin in-gel digestion and further analysis in a mass spectrometer. This technique of analysis is called Peptide Mass Fingerprinting, which provides a spectrum's peak list (masses of peptides). After that, the masses of peptides derived from an in-gel proteolytic digestion are measured and searched against a computer- generated list formed from the simulated digestion of a protein databases using the same enzyme.

     Up to this moment, 24 proteins were identified. Among these proteins, ten are related to the inflammatory process, acting as cytokines, proteases, vasoactive compounds, or protecting the neutrophil against oxidative damage. They are IL-1, IL-13r, TNFr, azurocidin, serine protease, actin-related-protein-2, actin-g-1, peroxiredoxin-2, autoimmune regulator and collagenase. Three structural proteins were identified: nucleophosmin, a nucleolar protein; chloride channel protein 5, an ion transporter and cytoskeleton keratin, a cytoskeleton intermediate filament. Interestingly, some of the identified proteins like Huntingtin-Interacting-Protein, IL-13r, peroxiredoxin-2 and collagenase were first described in neutrophils through this study.

     Although the human genome is well characterized, a large number of proteins still remain as hypothetical. Ratifying that, five previously described as hypothetical proteins were identified in the present study, being then considered as proteins with unknown function. The other proteins identified in this study are thiol antioxidant protein, pur beta human protein, supported protein of EST, phosphoglycerate mutase and an unnamed protein.