Comparison of expression of CD4 versus CD25 in human and mouse. Peripheral-blood leukocytes were stained with mAbs to CD4 and CD25, and gated for lymphocytes expressing CD4. (A) In mice, the number of conventional activated CD4⁺ T cells expressing CD25 was low compared with the number of regulatory CD4⁺CD25⁺ cells, even when a large number of CD4⁺ T cells were actively proliferating during reconstitution of lymphopenic host mice (“Activated” plots). (B) In adult blood, up to 20% to 30% of CD4⁺ T cells expressed CD25, and many of these cells appeared to be effector and memory cells resulting from encounter with foreign antigens. In contrast, the human cord-blood profile resembled that of mice.