The role of antibodies in immunity against Theileria annulata infection in cattle.

The role of antibodies in protecting cattle against Theileria annulata infection was investigated. It was found that serum samples from immune cattle had a neutralizing effect on the sporozoites preventing in this way the invasion of lymphocytes. However, the same antiserum neither caused lysis of macroschizont- infected lymphoblastoid cells nor inhibited their proliferation in vitro. Free merozoites but not the infected erythrocytes were opsonized by immune serum. In addition, complement alone, without antibody participation, was able to lyse free merozoites. The importance of humoral factors in immunity to T. annulata infection is discussed.