What makes psoriatic and rheumatoid arthritis so different?

In many ways, it may be easier to highlight what rheumatoid arthritis (RA) and psoriatic arthritis (PsA) have in common. They are both common conditions characterised by a spectrum of features or clinical manifestations in different organ systems that have led many to conclude that they are actually 'disease syndromes'. Furthermore, many of the organ systems that are affected in both conditions are the same: skin, joints, eyes, vasculature and even the immune system. Indeed, some clinicians fail to recognise these two common arthritides as distinct. And yet, while the manifestations may have a superficial similarity, there are significant differences at a number of levels including clinical, anatomical, microscopic and molecular levels. However, these differences may explain certain clinical manifestations of the two diseases, and more importantly, they may explain different responses to specific therapies and potentially different disease outcomes and prognoses. This may be especially relevant as new therapeutic targets are examined that may be specific for RA or PsA.