APCS and major depressive disorder: The proposed biological mechanism for increased SAP in depression is as follows: 1) Aβ increases through direct interaction with Aβ peptide at the carbohydrate-binding site of SAP; 2) central SAP, Aβ, and complement C1q attract and activate microglia; and 3) peripheral SAP regulates macrophage activation through Fcγ receptors34.