Kinetics of adaptive immune response following B microti infection in C57BL/6 mice. Groups of C57BL/6 mice were infected with 1 × 10⁷ to 1 × 10⁸B microti–parasitized RBCs intraperitoneally and a representative experiment is shown. (A) The frequency of peripheral blood erythrocytes infected with B microti in total number of RBCs over time. (B-D) Peripheral blood on the indicated days following B microti infection were analyzed using an Advia 120 hematology system for absolute numbers of RBCs (B), hemoglobin (Hgb) content (C), and reticulocyte percentages (D) (n = 3-10). (E-F) The spleen weights (E) and the number of splenocytes (F) before and after B microti infection (n = 3-12). (G) Representative flow cytometry (FACS) plots for GL7 and FAS gated on B220⁺ cells for the identification of splenic GC B cells (top panel) or B220 and CD138 for the identification of plasma cells (bottom panel) before infection (D0) and after infection on days 3, 10, and 14 (D3, D10, and D14). (H) Quantification of GC B cells (top panel) and plasma cells (bottom panel) (n = 3-10). (I-J) Flow cytometric analysis of T_FH cells in C57BL6 mice. (I) Representative FACS plots for CXCR5 and PD1 expression gated on splenic CD4⁺ T cells. (J) Quantification of T_FH cells (CD4⁺CXCR5^hiPD1^hi) (n = 3-10). Each symbol represents data from an individual mouse. Error bars indicate the mean ± standard error of the mean (SEM). *P < .05; **P < .01; ***P < .001; ****P < .0001 by unpaired 2-tailed Student t test.