Pyrocytes are a unique specialist cell within the body of an organism. Not much is known about the function of these cells but it is known that they cause a severe allergic reaction in Humans and other species if applied through the skin.
The Ferengi were known to possess such cells within their bodies and were used as a tactic during negotiations in order to remove certain opponents from the negotiation table.
Doctor Arridor used distillated pyrocytes from DaiMon Gross in 2366 to sicken Federation negotiator Seth Mendoza in order to remove an opponent from the negotiaton table during the discussion over the rights to the Barzan wormhole. (TNG episode: "The Price")
After the disaster in trying to incorporate Federation replicator technology into their ships, a Kazon crewmen suffered severe damage after his body partially bonded with metallic matter. This resulted in The Doctor attempting to completely replace the pryocytes within the Kazon's body and used a cytological scan of the USS Voyager's crew in order to find a suitable donor. (VOY episode: "State of Flux")