in - 1 5 ▁( IL - 1 5 ) ▁is ▁a ▁cy tok ine ▁with ▁struct ural ▁similarity ▁to ▁Inter le uk in - 2 ▁( IL - 2 ). ▁ ▁Like ▁IL - 2 , ▁IL - 1 5 ▁bind s ▁to ▁and ▁signals ▁through ▁a ▁complex ▁composed ▁of ▁IL - 2 / IL - 1 5 ▁re ceptor ▁beta ▁chain ▁( CD 1 2 2 ) ▁and ▁the ▁common ▁g amma ▁chain ▁( gamma - C , ▁CD 1 3 2 ). ▁ ▁IL - 1 5 ▁is ▁secret ed ▁by ▁mon on uc lear ▁ph ag ocy tes ▁( and ▁some ▁other ▁cells ) ▁following ▁in fection ▁by ▁virus ( es ). ▁ ▁This ▁cy tok ine ▁indu ces ▁the ▁pro l ifer ation ▁of ▁natural ▁k iller ▁cells , ▁i . e . ▁cells ▁of ▁the ▁inn ate ▁imm une ▁system ▁whose ▁principal ▁role ▁is ▁to ▁kill ▁vir ally ▁inf ected ▁cells . ▁ ▁Expression ▁▁ ▁IL - 1 5 ▁was ▁discovered ▁in ▁ 1 9 9 4 ▁by ▁two ▁different ▁labor ator ies , ▁and ▁character ized ▁as ▁T ▁cell ▁growth ▁factor . ▁T ogether ▁with ▁Inter le uk in - 2 ▁( IL - 2 ), ▁Inter le uk in - 4 ▁( IL - 4 ), ▁Inter le uk in - 7 ▁( IL - 7 ), ▁Inter le uk in - 9 ▁( IL - 9 ), ▁gran ul ocy te ▁col ony - st im ulating ▁factor ▁( G - CS F ),