IL17A and rheumatoid arthritis: Some researchers have found that the infiltration of memory CD4+ T cells is directly implicated in rheumatoid arthritis, which is a typical arthritis with inflammation and a high amount of CD4+ T cells always releases a high level of interleukin‐17 (IL‐17) and tumor necrosis factor‐β (TNF‐β), which are the most important inflammatory factors (Chemin et al., 2019).