CCL21 and neoplasm: Interestingly, CCR7/CXCR4 dimers may enhance tumor B-cell homing to LN by potentiating the TEM upon simultaneous exposure to CXCL12, CCL19, and CCL21 (90) while in healthy, mature B-cells CCR7 acts as a selective allosteric modulator that inactivates CXCR4 thus impairing retention in the BM (236).