The 3C7 monoclonal antibody specifically binds to CD25, the low affinity IL-2 Receptor (IL-2Rα, p55) expressed on activated T and B lymphocytes from all mouse strains tested. IL-2Rα by itself is not a signaling receptor. However, it can combine with IL-2 Receptor β (CD122) and γc (CD132) chains to form high-affinity signaling receptor complexes for IL-2. Resting T and B lymphocytes as well as resting and activated NK cells do not express IL-2Rα. CD25 is transiently expressed at a low level during normal B-cell development in the bone marrow during the CD45R/B220low TdT- sIg- Pre-B/Pre-B-II and CD45R/B220low TdT- sIgM+ sIgD- immature B stages, but not during the CD45R/B220low TdT+ sIg- Pro-B/Pre B-I stage nor on CD45R/B220high TdTsIgM+ sIgD+ mature B cells. It is expressed at a higher level during a very early stage of T-cell development in fetal and adult thymus. Peripheral CD25+ CD4+ T lymphocytes called regulatory T (Treg) cells are involved in the maintenance of self-tolerance. It has also been reported that dendritic cells express CD25 (recognized by mAb 7D4, another CD25-specific antibody). The 3C7 antibody recognizes an epitope of CD25 which is distinct from those recognized by the other CD25-specific mAbs, 7D4 and PC61. 3C7 blocks the binding of IL-2 to CD25.