The invention provides methods and compositions for exploiting the discovery that members of the small integrin-binding ligand, n-linked glycoproteins family termed SIBLINGS bind to complement Factor H, and moreover that SIBLINGS proteins, such as BSP, exist in relatively acidic forms. The methods provided can be used to detect SIBLINGS proteins in samples from subjects that are suspected of having tumors or abnormal bone turnover. The invention also provides methods of using SIBLINGS proteins to protect cells from complement mediated lysis. Finally, the discovery allows for the creation of specific binding agents that facilitate the detection of SIBLINGS proteins when they are associated with Factor H.