Charges play an important role in determining key solution properties of monoclonal antibodies (mAbs) such as the colloidal stability against aggregation, the viscosity and the occurrence of liquid-liquid phase separation. These properties are essential for creating high concentration formulations needed for self-administration via subcutaneous injections as the preferred application for mAb-based biologics. Here, we report the results of a systematic investigation of the solution properties of charged mAbs as a function of concentration and ionic strength using a combination of electrophoretic measurements, static and dynamic light scattering, small-angle x-ray scattering (SAXS), and tracer particle-based microrheology. We demonstrate that increasing patchiness with well-defined oppositely charged surface regions strongly alters the solution behavior of mAbs and can lead to high viscosity and the onset of liquid-liquid phase separation. We furthermore show that while colloid models have often been successfully used to describe charge contributions to protein-protein interactions for compact globular proteins, the anisotropic Y-shaped structure of mAbs combined with a complex and heterogeneous charge distribution requires much more refined models.