The chemokine (C-C motif) ligand 22 (CCL22), also known as macrophage-derived chemokine (MDC), is a member of the CC chemokine family. Cells known to express CCL22 include B cells, microglia, dendritic cells and macrophages, monocytes, IFN-γ-activated keratinocytes, colonic columnar epithelium, CD4 T cells, thymic medullary epithelium (Hassal’s corpuscles), and activated NK cells. CCL22 plays an important role in lymphocyte migration. CCL22 is a potent chemoattractant for cell types including dendritic cells, thymocytes, and activated NK cells. CCL22 produced by antigen-presenting dendritic cells chemoattracts CCR4-bearing activated (or memory) T cells to enhance immune responses and increase effector functions. Additional functions reported for CCL22 include the activation of platelet function and inhibition of synaptic transmission between hippocampal neurons.