ISG15 and infection: To determine if the increase of ISG15 during the early phase of Toxoplasma infection is caused by type I IFN, we infected IFNαR−/− mice with Toxoplasma type II and measured the level of ISG15 released in the serum 4 d p.i. As opposed to infection in C57BL/6 wild-type mice, infection of IFNαR−/− mice did not result in increased ISG15 serum levels.