CCL22, also known as ABCD-1 and MDC (macrophage-derived chemokine), is a CC chemokine. It is highly expressed in macrophage, monocyte-derived dendritic cell and thymus, additionally, also detected in the tissues of thymus, lymph node and appendix. CCL22 can bind to CCR4, and is a chemoattractant for monocytes, monocyte-derived dendritic cells, and natural killer cells, but not for neutrophils, eosinophils, and resting T-lymphocytes. After secreted from monocyte-derived dendritic cells, the protein can be proteolytic cleaved into three forms: MDC (3-69), MDC (5-69), MDC (7-69). CCL22 is a functional ligand for the CC chemokine receptor 4.