C3 and Alzheimer disease: Recent studies have implicated complement receptors such as CR3 (receptor for a C3-cleavage product iC3b), C3aR (receptor for anaphylatoxin C3a, also a cleavage product of C3), and C5aR (receptor for anaphylatoxin C5a, a cleavage product of C5) in the early synapse loss observed in AD.