TO STICK OR NOT TO STICK - THE NEW LEUKOCYTE HOMING PARADIGM

The immune system is formed by leukocytes. They are passively transpor ted through the body by the vascular system, but their entrance into t issues requires a coordinated series of events, namely activation of l eukocyte integrins, adhesion to the vascular endothelium, and migratio n. There are four steps in this process, which begin with the rolling of leukocytes along the vascular endothelium, followed by signaling wh ich activates leukocyte integrins, thus leading to tight adhesion to t he endothelium and finally transmigration. Substantial progress has be en made recently in elucidating the molecular events that induce rolli ng and signaling, partly as a result of the study of double-knockout m ice that are deficient for genes encoding two selectins.