The immune system produces proteins, known as chemokines, which are signals that can affect immune function. There is a chemokine named C-C motif ligand 20, also known as CCL20. It is known as an inflammatory protein, often associated with rheumatoid arthritis. However, new evidence links this cytokine with numerous cancers. It seems that CCL20 promotes cancer metastasis. In addition, it increases cancer stem cells and causes T cell exhaustion. These actions will suppress an immune response. There are studies in many cancer types indicating that blocking CCL20 is anti-cancer and may enhance the success of immunotherapy. Surprisingly enough, drugs that can block CCL20 may also reduce autoimmune side-effects from standard PD-1 immunotherapy, such as Opdivo and Keytruda.