CD38 and rheumatoid arthritis: In conclusion, our comparative study assessing the histological features of synovial tissues obtained from naive to treatment and MTX-IR PsA and Abneg RA patients revealed that PsA synovitis is characterized by being mast cell (CD117+) rich but plasma cell (CD138+) poor whereas Abneg RA synovitis by the reverse findings, being plasma cell (CD38+) rich but mast cell (CD117+) poor.