Platelets, or thrombocytes, were a type of blood cell mainly involved in hemostasis, the formation of blood clots.
Genetically engineered Human Augments created on Earth in the 20th century had powerful platelets in their blood, allowing enhanced cell regeneration. When regular Humans or Tribbles were injected with augment platelets fatal cellular injuries such as lethal radiation exposure could be healed. The platelets could even resuscitate a dead Tribble back to life. (Star Trek Into Darkness)