Structure of eculizumab. Eculizumab was engineered to reduce immunogenicity and eliminate effector function. Human IgG2 and IgG4 heavy-chain sequences were combined to form a hybrid constant region that is unable to bind Fc receptors or to activate the complement cascade. Eculizumab exhibits high affinity for human C5, effectively blocking its cleavage and downstream proinflammatory and cell lytic properties. Reprinted from Rother et al³⁶  with permission.