Major acute phase reactant. Apolipoprotein of the HDL complex. In vitro exhibits antimicrobial activity against Escherichia coli, Streptococcus uberis and Pseudomonas aeruginosa (By similarity). Expressed by the liver; secreted in plasma. Expressed in synovial fibroblasts (PubMed:1849144). Up-regulated by cytokine stimulation. 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.