The present invention concerns determination of the crystal structure of the macrophage specific receptor, CRIg (earlier referred to as STIgMA), and its complex with the C3b and C3c subunits of complement C3 (C3b:CRIg and C3c:CRIg complexes). The invention further concerns the use of the crystal structure of CRIg or the C3b:CRIg complex to screen for and identify molecules structurally and/or functionally related to CRIg, including CRIg agonists and antagonists.