Eculizumab specifically binds to the terminal Complement component 5, or C5, which acts at a late stage in the complement cascade.
Eculizumab inhibits the cleavage of C5 to C5a (a potent anaphylatoxin with prothrombotic and proinflammatory properties) and C5b by the C5 convertase, which prevents the generation of the terminal complement complex C5b-9 (which also has prothrombotic and proinflammatory effects).