PDCD1 and infection: To establish whether PD-1 signaling modified long-term B cell responses, cohorts of WT or PD-1KO mice were infected with Pch pRBC, rested for 15 weeks after the clearance of patent primary parasitemia (~35–40 days) and then re-infected with the same dose of Pch. Sera from these mice were collected a day before and 5 days after re-infection, and tested by ELISA for parasite-specific IgG antibody titers.