At homeostasis the eosinophil contributes 1 -4 percent of the circulating leukocyte pool. ECP is readily detectable in blood with plasma levels on the average 3 ug/L (serum 7 μg/L) in healthy individuals which correlates with circulating eosinophil numbers [43] . ECP in blood shows a turnover time (t 1/2 ) of 45 min [44] , and the plasma protein α 2 -macroglobulin (α 2 M) is found to be associated to the protein, in vitro at a molar ratio of 1.6 (ECP/α 2 M). This interaction is facilitated by proteolytic activity of cathepsin G or methylamine [45] , and conceivably takes place to facilitate the clearance of ECP [46] .