Schematic representation of the Hp and Hpr structures. Using electron microscopy, Wejman et al49 described the Hp1-1 molecule as a barbell-like elongated structure with 2 spherical head groups (the β-chains) connected by a thin filament with a central knob (the α-chains). The disulfide bridge pattern of Hp1-1 and Hpr is indicated. Examples of structures of the Hp2-1 and Hp2-2 molecules, as reported by electron microscopy by Wejman et al,48 are also shown.