Given that differences in synovial cell infiltrate between RA and spondyloarthropathy (SpA) have been described, with significantly more mast cells [13], neutrophils (innate immunity) and endothelial cells in SpA [14], the objective of this study was to analyze whether these cells are also overrepresented in ACPA negative RA patients, and whether cells related to adaptive immunity (T and B cells and lymphoid neogenesis structures) are more abundant in ACPA positive patients. The gene discussed is PRTN3; the disease is rheumatoid arthritis.