CCR7 and infection: In addition, whereas naïve T and B lymphocytes require the chemokine receptor CCR7 to activate their LFA-1, arrest on HEVs and cross these vessels, following infection, additional signals such as CXCR3 chemokines may provide critical α4 integrin activating signals that allow these lymphocytes to overcome the absence of ICAMs on MedLNs and use their α4 integrins to arrest on HEVs expressed VCAM-1 and MadCAM-1.