Cañete et al. [134] reported that retinoic acid is involved in the homeostasis regulation, T cell differentiation, movement of T cell migration into tissues, antibody-dependent T cell development, proliferation and differentiation of B cells, B cells protection from apoptosis by binding with toll-like receptor 9 (TLR9), modulation of granulocyte, neutrophils differentiation, and treatment of cancer. Here, TLR9 is linked to cancer.