Activated anti-MSP-1 T cells help immune B cells make a faster and greater antibody response, which is critical for clearance. B5 (▪) and BALB/c (□) CD4⁺ cells were purified by high-speed flow cytometry and transferred into RAG^-/- mice, which were then infected with 10⁴P chabaudi (AS) parasites (A-C). Some mice were immunized with the T-cell antigen (MSP1_900-1507) covalently linked to the B-cell antigen (MSP1₂₁) intraperitoneally 2 days after T-cell transfer and a day before infection (D-F). Antibodies recognizing MSP1_900-1507 (A-B), MSP1_1508-1766 (B,E) and whole parasite lysate (C,F) were measured by ELISA. The results are expressed as arbitrary units of antibody relative to a standard hyperimmune serum. The values shown are the geometric means and SEs of IgG responses of 5 to 7 mice. The asterisk indicates that the differences are significant (P < .05, Student t test). (G) T cells protect from lethal infection but a threshold of antibody is critical in clearance of P chabaudi. A schematic representation of the influence of transferred transgenic CD4⁺ T cells and immune B cells in the presence and absence of immunization on the rate of clearance of P chabaudi infection (▪), mortality of RAG^-/- mice (□), and the speed and magnitude of a malaria-specific antibody response (gray shaded area).