Female BALB/c mice, SPF grade, 6–8 weeks old with body weight of 18–22 g were used for the study, and guinea pigs (250 g) were used for the preparation of complement for plaque-forming cell assay and hemolysis test. The animals were purchased from Beijing HFK Bioscience Co., Ltd. (Beijing, China). The animals were maintained in a controlled environment at a temperature of 20–25°C, relative humidity of 40–70%, artificially illuminated with a 12 hour light/dark cycle and air exchanges of 10–15 times per hour. All animals were examined for clinical signs of ill health on receipt and observed within 7 days of arrival. The study was approved by Animal Experimental Welfare & Ethical Inspection Committee, National Institute for Nutrition and Food Safety, Chinese Center for Disease Control and Prevention. Animal experiments and housing procedures were carried out in accordance with the laboratory animal administration rules of the Ministry of Science and Technology of the People's Republic of China.