Interleukin-7: an interleukin for rejuvenating the immune system.

Infection of an individual (aged 20-30 years) by a virus will cause a response from the T (thymus derived) lymphocytes of which there are approximately 3 x 10(11). If the individual has not met the virus before, the response will come from the naive T cell subset (50 +/- 10% of the total T cell pool at this age) containing recent thymic emigrants produced from the thymus at approximately 10(8) per day. Their antigen-specific receptor has a defined specificity governed by the conformation of its two chains (alpha and beta), and the repertoire of specificities is somewhere in the region of 2 x 10(7) to 10(8). A successful response leads to clonal expansion and the generation of memory T cells to the infecting agent.