Since the discovery of ECP, assays have been developed to determine its levels in biological fluids in various diseases (table 1) . ECP in serum can reach 0.1 -0.2 μg/mL for HES patients [111] and parasitic diseases infected individuals [72] and this is a 30 fold elevation compared to ECP in serum of healthy individuals. In BALF and nasal lavage from atopic patients the ECP levels are lower, 0.050 μg/mL but the sample are diluted during the collection process. In undiluted tears, sputum and nasal secretions the highest ECP levels have been found: 0.5, 0.7 and 10 μg/mL, respectively. The ECP measurements correlate with eosinophilic disease but have been found elevated also in some diseases without known eosinophil involvement [76] [77] [78] . The biological activity of ECP has been studied by incubation of the protein with several different cell types in vitro. Both human cells and pathogens have been assayed analysing different parameters (see table 2 ). In general, 10 -20 μg/mL and above, result in growth inhibitory and destructive consequences to mammalian cells, parasites and bacteria. ECP released in situ in diseases engaging high levels of eosinophils might reach these destructive concentrations (e.g. ECP accumulated in air way mucus of asthmatics, in nasal secretions of some sinusitis patients or released in skin of atopic dermatitis/HES patients, table 1 and Figure 5 ). Although it remains to be proven, there is a possibility that destructive activity to multiple cell types as well as induction of fibrosis is part of the etiology of disease where ECP levels are elevated during prolonged periods, e.g. in HES and helminth infection. There is also evidence that neutrophils are carriers of significant amounts of ECP. Using the murine system, granule proteins have been found associated with expelled eosinophil mitochondrial DNA and this DNA/protein complex trapping and killing bacteria in the gut [134] . It is intriguing to speculate whether the high levels of ECP present in various human mucosal secretions would equally be associated with eosinophil mitochondrial DNA and whether such complexes had the ability to capture and kill microorganisms.