Interleukin-2 (IL-2) was the first of the immunologic cytokines to be discovered and characterized and it remains the most therapeutically useful of the interleukins so far discovered. While about a dozen interleukins are now recognized, several of these do not appear to be directly involved with immunity. IL-2, on the other hand, is crucial for the generation of an effective immune response. After an antigen binds receptors on an individual T-cell , the antigen stimulates the T-cell to secrete IL-2 and to make IL-2 receptors. IL-2 has been found to be produced mainly by the T helper cell . Within 24-48 hours of activation, T helper cells begin to synthesize and secrete IL-2 and to express high affinity membrane receptors for IL-2. The IL-2 receptor acts as an on-off switch. It has two sites - the first site readily binds IL-2, while the second site attaches more slowly to the IL-2 molecule. It is the interaction at the second site that activates the T-cell, causing it to undergo complex changes in morphology, metabolism, expression of surface receptors and the production of cytokines. The activated T-cell starts to synthesize DNA and divides, producing two T-cells that can now be activated by antigen. This proliferation continues until there is a clone of identical T-cells, each capable of binding antigen. Thus, activated T-cells that produce IL-2 can :

1. Promote their own clonal expansion
2. Promote the proliferation of other T-cells that are activated by the same or related specific antigen but cannot produce IL-2 (i.e. CD8+ cells).
3. Promote the expansion of previously stimulated cells that express low levels of high-affinity IL-2R (i.e. memory T-cells)
4. Do not influence unstimulated T-cells because those T-cells do not express a high affinity IL-2 receptor .