Attenuated immunogenic parasites are essential in the transfer of immunity to virulent Plasmodium berghei.

A less virulent parasite of Plasmodium berghei K173 was isolated that induced immunity against the more virulent parasite. Immunity to this parasite but not to the virulent one, could be transferred by immune spleen cells but not by immune lymph node cells. However, the immune spleen cells did transfer immunity to the virulent strain if accompanied by infection with viable parasites of the less virulent strain, but only if they were allowed to proliferate for a period of 1 week before challenge with the virulent strain. Immune spleen cells could survive two cycles of mouse to mouse transfer. The induction of immunity by transfer of immune spleen cells was associated with the production of anti-parasitic antibody.