Methionine enkephalin: a new cytokine--human studies.

The effects of methionine enkephalin (met-enkephalin) on human immune function are reviewed. This pentapeptide functions to upregulate, or enhance, immune function in the majority of donor samples at low doses and suppresses at high doses. The influence of this molecule is shared by the central nervous, neuroendocrine, and immune systems. Cells from each of these systems possess receptors for met-enkephalin and have the ability to process met-enkephalin from its prohormone, proenkephalin A. Studies have shown that this molecule is capable of enhancing immune function in patients with cancer or AIDS. It is proposed that this molecule be classified as a cytokine.