Traffic Signals on Endothelium for Lymphocyte Recirculation and Leukocyte Emigration

The circulatory and migratory properties of white blood cells have evolved to allow efficient surveillance of tissues for infectious pathogens and rapid accu­ mulation at sites of injury and infection. Lymphocytes continually patrol the body for foreign antigen by recirculating from blood, through tissue, into lymph, and back to blood. Lymphocytes acquire a predilection, based on the environment in which they first encounter foreign antigen, to home to or recirculate through that same environment (39, 40). Granulocytes and mono­ cytes cannot recirculate, but emigrate from the bloodstream in response to molecular changes on the surface of blood vessels that signal injury or infec­ tion. Lymphocytes can similarly accumulate in response to inflammatory stim­ uli. The nature of the inflammatory stimulus determines whether lymphocytes. monocytes, neutrophils, or eosinophils predominate, and thus exercises spec­ ificity in the molecular or codes that are displayed on endothe­ lium and control of particular leukocyte classes. Recent findings show that the traffic signals for lymphocyte recirculation and for neutrophil and monocyte localization in inflammation are strikingly similar at the molecular level. These signal or area code molecules are