CXCR4 and neoplasm: Chemokines and their receptors play an important role in directing cognate T-APC interactions in lymph nodes and spleen (e.g., CCR7 and CCL21/19), or in bone marrow (e.g., CXCR4 and CXCL12 (SDF-1α/β), or attracting Treg cells towards tumor tissue (e.g., CCR2 and CCL2 (MCP-1)).