OBJECTIVE To study the early immune activation and its dynamic changes between the attenuated cercariae immunized mice and the normal infected mice. METHODS The dendritic cell surface molecules CD11c and T cell surface molecule CD25 expression differences and CD3+CD25+/CD3+ T ratio of the early spleen and/or lung of the attenuated cercariae immunized mice and normal mice were assayed and compared by FCM and IHC, and the immune activation and dynamics of T cells were analyzed. RESULTS CD3+CD25+CD3+ T ratio in the spleen cells 7 days post-infection in the immunized group and the normal infected group were (19.52 +/- 3.65)% and (22.12 +/- 3.24)%, respectively; the rates of 14 days and 21 days post-infection were (28.73 +/- 3.94)%, (13.68 +/- 3.64)% and(26.43 +/- 0.40)%, (14.42 +/- 2.24)%, respectively. The expressions of CD11c+DC in the lung of the two groups were (1.05 +/- 0.16)%, (0.96 +/- 0.15)%, (1.34 +/- 0.15)%, (1.09 +/- 0.17)%, (1.49 +/- 0.14)%, (0.97 +/- 0.16)%, respectively; the expressions in the spleen were (2.05 +/- 0.26)%, (1.95 +/- 0.18)%, (2.24 +/- 0.25)%, (2.17 +/- 0.25)%, and (2.18 +/- 0.26)%, (2.06 +/- 0.18)%, respectively, on the 7, 14 and 21 days post-infection. The expressions of CD25+T cells in the lung of the two groups were (1.24 +/- 0.13)%, (1.17 +/- 0.16)%, (1.48 +/- 0.11)%, (1.25 +/- 0.13)%, and (1.55 +/- 0.14)%, (0.97 +/- 0.12)%, respectively; the expressions in the spleen were (3.25 +/- 0.22)%, (2.93 +/- 0.20)%, (4.57 +/- 0.23)%, (3.69 +/- 0.24)% and (4.28 +/- 0.24)%, (3.86 +/- 0.26)%, respectively, on the 7, 14 and 21 days post-infection. The CD3+CD25+/CD3+T rate in the infection control group was significantly higher than that in the cercariae attenuated group, while 14, 21 days post-infection the rates of the attenuated group were significantly higher than those in the normal control group. On the 7, 14 and 21 days post-infection, the lung tissue of the attenuated cercariae immunized mice raised more CD11c+ DC and CD25+ T cells than that of the normal infected mice did. CONCLUSIONS The activation of T cells of the immune group and the activation of pulmonary dendritic cells are higher than those in the control group 7 and 14 days post-infection, suggesting that attenuated cercariae in the lungs can raise more antigen presenting cells and their activation.