Zinc as a signal molecule for immune cells. Zinc homeostasis is tightly controlled by three mechanisms: (A) Transport through the plasma membrane by zinc transporters from the ZnT (SLC A30) or ZIP (SLC A39) families. (B) Buffering by metallothionein. (C) Reversible transport by ZnT and ZIP proteins into or out of zincosomes, and storage bound to ligands that form a zinc sink. Zinc signals, i.e., changes in the intracellular concentration of free zinc, control immune cell signal transduction by regulating the activity of major signaling molecules, including kinases, phosphatases, and transcription factors. One representative example for each group is given. (TCR, T cell receptor; MKP, MAPK phosphatase; MTF-1, metal-response element binding transcription factor-1).