Complex sentence: Ehrlich interpreted this as immunization and observed that it was abruptly initiated after a few days and was still in existence after several months, but mice immunized against ricin were just as sensitive to abrin as untreated animals.
Split into simpler sentences: Ehrlich interpreted this as immunization and observed that it was abruptly initiated after a few days and was still in existence after several months. But mice immunized against ricin were just as sensitive to abrin as untreated animals.