FAP and rheumatoid arthritis: Therefore, as a surface marker of RA-FLS, FAP plays an important role in initiating abnormal immune responses and promoting RA development by mediating the interaction between RA-FLS and immune cells, regulating the differentiation, migration, and invasion of RA-FLS, facilitating the secretion of various cytokines, relieving contact inhibition, and inhibiting necrotic apoptosis (19, 34, 35).