Acylation Stimulating Protein (ASP) is one of the factors that control adipose tissue function. It was initially identified from human plasma, and determined to be a derivative of the 3rd complement component (C3). Arginine removal from activated C3 (C3a) by carboxypeptidase results in C3a-desArg, also known as ASP.

Human fat cells have been shown to have mRNA for Adipsin (complement factor D), C3 and factor B, all of which are components necessary to generate C3a, the precursor for ASP.  Basal production of ASP in adipocytes is dependent on adipocyte differentiation, with ASP production increasing after 7 days of adipocyte differentiation. While ASP is produced by the interaction of C3, factor B and Adipsin in the alternate complement pathway, it can also be potentially generated by the classical complement pathway as well as the lectin pathway.