Major acute phase reactant. Apolipoprotein of the HDL complex. Expressed by the liver; secreted in plasma. Upon cytokine stimulation. At least 2 different SAA2 alleles have been described: SAA2.1 (SAA2alpha) and SAA2.2 (SAA2beta). We use here the revised nomenclature described in PubMed:10211414. The sequence shown is that of SAA2.2. Reactive, secondary amyloidosis is characterized by the extracellular accumulation in various tissues of the SAA2 protein. These deposits are highly insoluble and resistant to proteolysis; they disrupt tissue structure and compromise function. Belongs to the SAA family.