Snails are the intermediate host of Schistosoma and they play an important role in the transmission of schistosomiasis. The snails in different areas present various susceptibilities, which are related to the different hemocytes in the snails. There is not a common standard in the classification of hemocytes, and generally the hemocytes are divided into two categories: a granulocyte and a hyalinocyte. The granulocyte plays an important role in immunization, while hyalinocyte shows a less effect. Besides, soluble factors may also play a role in immunization.