This book is all about the physiological and pathophysiological functions of vascular endothelium and CD73 (ecto‑5’-nucleotidase), an ecto enzyme catalyzing the formation of adenosine from AMP. The book also highlights the methodology to analyse the membrane protein composition in endothelial cells (EC) and regulatory T‑cells using Mass Spectrometry (MS) based techniques under control conditions and in the absence of CD73. It answers the question if lack of CD73 causes secondary changes in the composition of membrane proteins of ECs and regulatory T‑cells. The membrane protein composition on ECs was determined using the colloidal silica beads method with MS based techniques. Stable isotope dimethyl labeling method was employed to investigate the quantitative and differential proteome analysis in regulatory T cells isolated from control and CD73 knockouts. The main content of this book includes proteomic analysis of EC membranes under in vivo conditions; functional role of endothelial CD73 (ecto- 5’-nucleotidase); limitations of proteomic analysis of ECs; functional role of CD73 in regulatory T cells (T reg) and differentially expressed proteins in T reg cells lacking CD73.