Involved in lymphocyte proliferation and functions as a signal transmitting receptor in lymphocytes, natural killer (NK) cells, and platelets. Homodimer; disulfide-linked. Expressed on the surface of activated T-cells, B-cells, natural killer cells, neutrophils, eosinophils, epidermal Langerhans cells and platelets. Earliest inducible cell surface glycoprotein acquired during lymphoid activation. By antigens, mitogens or activators of PKC on the surface of T and B-lymphocytes. By interaction of IL-2 with the p75 IL-2R on the surface of NK cells. Constitutive Ser/Thr phosphorylation in both mature thymocytes and activated T-lymphocytes. CD69