Endothelial cells of the vascular inner lining, in addition to their barrier functions, play certain regulatory roles. They regulate the blood flow, selective permeability of the vascular walls, blood fluidity, hemostasis, and angiogenesis. Regulation of these physiological functions is mediated by the production of vasoactive molecules and cytokines. Endothelial cells are directly involved in leukocyte migration and recruitment from vessels into tissues, as well as into regions affected by infection and/or inflammation. Under certain conditions, they serve as antigen-presenting cells, regulate activation and differentiation of blood mononuclears, and determine specific immune responses. Intercellular mediators (cytokines) control these immunological functions.