Interleukin-2 (IL-2) was the first of the
immunologic to be discovered and
characterized and it remains the most therapeutically useful of the
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 binds on an individual
, 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 . 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 of identical T-cells, each capable of binding antigen.
Thus, activated T-cells that produce IL-2
- Promote their own clonal expansion
- Promote the proliferation of other T-cells that
are activated by the same or related specific antigen but cannot produce IL-2 (i.e.
- Promote the expansion of previously stimulated cells that express low levels of high-affinity IL-2R
(i.e. memory T-cells)
- Do not influence unstimulated T-cells because those T-cells do not express a
high affinity IL-2 .