PDPN and rheumatoid arthritis: Because FDCs are known to play a crucial role in the regulation of humoral immunity through the long‐term persistence of antibody production,24 the results of our study demonstrate activation of humoral immunity, and the number of CD21+, l‐caldesmon+, and D2‐40+ FDCs was significantly increased in FTH‐RA group compared with FTH‐MG group.