S100A9 and rheumatoid arthritis: S100A9 belongs to the family of damage-associated molecular patterns (DAMPs) (Ehrchen et al., 2009), which promote leukocyte aggregation through a positive feedback regulatory mechanism when the body undergoes an inflammatory response and are often used as a reliable marker for a variety of inflammatory diseases, such as psoriasis, cancer, and rheumatoid arthritis (Christmann et al., 2020; Gebhardt et al., 2006; Huang et al., 2019a).