CD4⁺T cells expressing cytokines and the kinetics of plasma immunoglobulin levels following B microti infection in C57BL6 mice. (A) Representative FACS plots for CD44 and IL21 (top) or IFNγ (bottom) expression gated on CD4⁺ T cells in the spleens from mice before (D0) and after B microti infection on days 5 and 10. (B-C) Plasma immunoglobulin levels of IgM (B), IgG1, IgG2b, IgG2c, IgG3, IgA, and IgE (C) on days indicated following Babesia infection. The concentrations of each immunoglobulin isotype were determined by Luminex assay with appropriate immunoglobulin standards. (D) The kinetics of B microti–specific antibody responses in C57BL/6 mice following infection. Plasma was collected from mice on the days indicated and the levels of IgM and IgG antibodies were determined by ELISA using crude cell lysate of RBCs infected with B microti. Each symbol represents data from an individual mouse. Error bars indicate the mean ± SEM. *P < .05; **P < .01; ***P < .001; ****P < .0001 by unpaired 2-tailed Student t test.