Abstract: By now, very little is known about events occurring during early infection and innate immunity to infection with E. granulosus following ingestion of the infective egg stage and establishment of the primary cyst. In this study, ten lambs of 4 - 6 months old of mixed sexes were divided into 2 groups of 5 lambs (one test and one control group). Each lamb in test group received 2000 intraperitoneally protoscolices and each lamb in the control group was injected only PBS. Sera were collected every two weeks until 24 weeks after challenge from each group and serum antibodies were tested by ELISA. Also white blood cells were measured every 2 weeks during this trial. All lambs were killed after 6 months and the internal organs were inspected for hydatid cysts. The results showed that the production of antibody was higher in three lambs of the test group in comparison with control group. In these lambs, hydatid cysts were observed in internal organs. The number and also the average size of hydatid cysts in these lambs were different. In two lambs of the test group, the level of antibody was increased gradually for a few weeks and was then constant. No cyst was observed in these lambs. The numbers of leukocytes including: eosinophils, lymphocytes, neutrophils and monocytes were increased in all of the lambs challenged with protoscolices in comparison with control group. The number of these cells was higher in lambs of test group with no cyst. In conclusion, this study showed that the production of antibody is not able to completely to protect against hydatid cyst and humoral immunity might be correlated with susceptibility to disease in lambs.

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