PRG4 and rheumatoid arthritis: Next to the degradation of factors responsible for the physiological functions of SF, such as lubricin or hyaluronan, by enzymes or radicals released upon local inflammation (57, 58, 59), also an enhanced influx of plasma consequent to the increased permeability contributes to the reduced concentrations of these factors observed for SF of RA patients (60), eventually diminishing the physiological functions of SF and likely resulting in impaired joint functioning.