To determine the in vivo relevance of T-bet+ B cells in malarial anaemia, we infected Ifngr1−/−, Tbx21−/− and Tlr9−/− mice with P. yoelii and followed the levels of splenic CD11c+ T-bet+ B cells, anti-PS IgG antibodies and the development of anaemia. This evidence concerns the gene TLR9 and anemia (phenotype).