Effect of 2 Gy on splenic white pulp and number of splenic MZB cells in C57BL/6 nude mice. (A) Overview of HE and IgM stainings, performed on frozen sections (all pictures taken at the same magnification): spleens from C57BL/6 nude mice before and 1 week and 4 weeks after 2 Gy TBI irradiation. The marginal zones are indicated by asterisks, showing that IgM⁺ MZB cells disappeared by 1 week after TBI and exhibited partial repopulation by 4 weeks after TBI. See “Light microscopy, immunohhistochemistry, and electron microscopy” for complete image acquisition information. (B) Flow cytometric analysis of splenocytes from C57BL/6 mice before and 1 week or 3 weeks after 2 Gy TBI irradiation. Results shown were obtained in the B220⁺ lymphocyte gate, MZB cells were identified as CD21^hiCD23^lo, follicular B (FO) cells as CD21^loCD23^hi, and B1b cells as CD21^negCD23^neg.