Test Details: CD 25 is considered as an activation antigen,also called IL-2 receptor alpha chain. CD4+ CD25+ cells are regulatory T cells (Treg) that suppress the activation of self-reactive T cells and prevent autoimmunity, may prevent cytotoxic T cells from killing tumour cells. It is, thus, used as a marker for hairy cell leukaemia and systemic mastocytosis in flow cytometric analysis.