Plasma Ig level was determined according to the method described by Siwicki and Anderson (1993). The oxidative radical production by phagocytes during respiratory burst was measured using the NBT(nitro-blue-tetrazolium) assay described by Anderson and Siwicki (1995). A turbidimetric assay was used to determine serum lysozyme levels using the method described by Hultmark et al. (1980) with slight modifications. Serum MPO activity was measured according to Quade and Roth (1997). SOD activity was measured by the percentage reaction inhibition rate of enzyme with WST-1 (water soluble tetrazolium dye) substrate and xanthine oxidase using a SOD Assay Kit (Sigma, 19160) according to the manufacturer's instructions. Serum antiprotease activity was measured according to the method described by Ellis et al. (1990) with slight modifications (Magnadóttir et al., 1999).