CD4 and rheumatoid arthritis: Persistently activated T cells play a pivotal role.[4] High‐throughput unbiased technologies, such as mass cytometry and single cell RNA sequencing (scRNA‐seq), have revealed that among T cells, the programmed cell death protein 1 (PD‐1) positive T cells, including the CXCR5+ CD4+ T cells (follicular helper T cells, TFH) and CXCR5− CD4+ T cells (peripheral T helper cells, TPH), are highly expanded in the inflamed synovium of patients with RA.[5] Under normal physiological conditions, immune responses are monitored by immune checkpoints to prevent autoimmunity.