Abstract
Thymocyte differentiation obeys the same fundamental principles in mammals as in avian
species. This parallelism does not only affect the developmentally controlled acquisition of
CD3, 4, 8, and TcR isotype expression, but also concerns CD25, the light chain of the interleukin-2 receptor (IL-2R). On chicken thymocytes, surface CD25, which is recognized by the
monoclonal antibody INN Ch16, is first observed during day 11 of embryonic life, and peaks
at day 14, when it is expressed by about one-third of all lymphoid cells. CD25 is found on
subsets of all ,thymocyte populations as defined by TcR