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The interleukins comprise a class of hormones that have a definite known secondary and tertiary structure under physiologic conditions. The interleukin-2 is the leader in T cell differentiation. The interleukin-2 acts via interaction with high affinity, cell bound receptors (IL-2R). High affinity IL-2 receptors are constructed by cooperative binding of IL-2 to both the low affinity (55-Kd chain) and intermediate affinity (75-Kd chain) binding sites. The light (55-Kd) chain of these heterodimeric receptors is identified by monoclonal antibodies as TAC antigen. A soluble form of these receptors is released in the serum and it can be assayed by ELISA. Extraordinarily high levels of IL-2R are characteristic of hairy cell leukaemia. Smaller increases of IL-2R have been reported in other haematological conditions as well as in other disorders including AIDS, organ transplantation etc. Moreover, we have recently demonstrated that IL-2R is elevated in lung cancer.