heat shock protein. (heet shok PRO-teen) Family of proteins found in all cells that are expressed in response to cold, heat and other environmental stresses, although some are constitutively expressed. HSPs increase heat tolerance and perform functions essential to cell survival under these conditions. Some serve to stabilize proteins in abnormal configurations, and play a role in folding and unfolding of proteins, acting as molecular chaperones. There are four major subclasses: HSP90, HSP70, HSP60 and small HSPs. HSPs are believed to act as major immunogens in many infections and disease states. Learn more about heat shock proteins.