HSP are groups of stress-inducible proteins which contribute to quality control by assisting the correct folding of both nascent and denatured proteins, and promoting the degradation of unrecoverable denatured proteins. HSP also help to maintain cellular homeostasis and protect from cell death through a mechanism called thermotolerance. Cells subjected to mild stress induce HSP which then protect them against subsequent stress. However, in cells subjected to severe stress, HSP promote apoptosis. Besides these intracellular events, HSP also exist in extracellular fluids, and have been shown to contribute to immunomodulation. In innate immunity extracellular HSP, like various microbial substances, induce various proinflammatory cytokines. In acquired immunity they interact with antigenic polypeptides and assist in antigen presentation. The extracellular HSP are so-called adjuvant. Release of HSP from cells is triggered by stress and trauma, and is thus regarded as an immunological “danger signal”. In addition, anti-HSP autoantibodies are frequently found in patients with autoimmune diseases and inflammatory disorders, and these autoantibodies can modulate the “danger signal” triggered by extracellular HSP.