This book discusses the mechanisms leading to immune-mediated tissue rejection following the hypothesis that independent of the disease process the final effector mechanism is shared by most (but not all) pathologies and it is relatively simple. The book covers evidence gathered to support the thesis by studies performed in humans during rejection or in experimental models and will focus particularly (but not exclusively) on the analysis of the rejected tissue rather than the systemic circulation. Several disease processes are discussed including example of chronic inflammatory process without resolution of the pathologic process and acute one with resolution of the pathologic process (clearance of pathogen, rejection of tumor) or unwanted tissue destruction (allograft rejection, autoimmunity).