Major acute phase reactant. Apolipoprotein of the HDL complex (By similarity). May have a role in protection of the mammary gland during remodeling and infection (Probable). In vitro exhibits antimicrobial activity against Escherichia coli, Streptococcus uberis and Pseudomonas aeruginosa (PubMed:19283521). Expressed in the liver (PubMed:19283521). Expressed in mammary epithelial cells (PubMed:11048944, PubMed:16837143, PubMed:19283521). Expressed at high levels in mammary ductal cells and vesicle engorged alveoli, but absent from stromal and connective tissue and leukocytes (PubMed:19283521). Secreted into colostrum and mastitic milk (at protein level) (PubMed:16837143, PubMed:19283521). Low expression levels, if any, in normal milk (at protein level) (PubMed:16837143, PubMed:19283521). Expressed at a moderate level in late pregnancy, at a low level through lactation, induced early in milk stasis, and expressed at high levels in most mammary epithelial cells during mid to late involution and inflammation/mastitis. Up-regulated by Gram-negative bacterial lipopolysaccharide (LPS) and the Gram-positive bacterial lipoteichoic acid (LTA) in mammary epithelial cells and to a lesser extent by prolactin/PRL. Belongs to the SAA family.