Major acute phase protein. Homohexamer; dimer of trimers. Can form amyloid fibrils after partial proteolysis; the native, undenatured protein does not form amyloid fibrils (in vitro). Apolipoprotein of the HDL complex. Binds to heparin (By similarity). Detected in liver, spleen and kidney. Up-regulated by inflammatory stimuli. Reactive, secondary amyloidosis is characterized by the extracellular accumulation in various tissues of the SAA protein. These deposits are highly insoluble and resistant to proteolysis; they disrupt tissue structure and compromise function. Belongs to the SAA family.