CCR9 and rheumatoid arthritis: Macrophages and DCs are the predominant sources of RA for B and T cells and via producing RA, can induce the expression of the gut‐homing adhesion molecules, including α4β7 integrin (also referred to as lymphocyte Peyer patch adhesion molecule; LPAM) and C‐C chemokine receptor 9 (CCR9) on B and T cells (Figure 3) [54, 55].