Using double immunofluorescence confocal microscopy, we identified EP1 and EP4 also in high populations of mature B-lymphocytes such as plasma cells within synovium of RA patients extending previous studies by Fedyk et al. [44] which demonstrated mRNA encoding EP1, EP3, and EP4 receptors in normal and transformed B-lymphocytes. This evidence concerns the gene PTGER4 and rheumatoid arthritis.