Interleukins are a group of cytokines ( secreted proteins and signal molecules ) that were first saw to be expressed by white blood cells ( leukocytes ) . Interleukins are a group of cytokines ( secreted proteins and signal molecules ) that were first seen to be expressed by white blood cells ( leukocytes ) . Interleukins are a group of cytokines ( expressed proteins and signal molecules ) that were first seen to be secreted by white blood cells ( leukocytes ) .