This section of the tutorial covers the main findings used to make the diagnosis of acute cellular rejection. It also has a brief word on antibody mediated rejection, which will be covered in greater detail in a forthcoming tutorial. It will consolidate the key concepts of the low power and high power sections.
A lymphocytic infiltrate is the key histologic finding that indicates the process being observed is acute cellular rejection. At low power, this is seen as an increased area of cellularity. At high power lymphocytes must be evaluted for their injury to myocytes. Activated lymphocytes, which are associated with injury can have enlarged nuclei and increased cytoplasm compared to quiescent lymphocytes.
Myocyte injury is the key feature to discriminate between low and high grades of rejection. Higher grades of rejection are 2R and 3R. Myocyte injury is best appreciated at high power where a variety of features can be noted. These include hypereosinophilia, nuclear pyknosis, myocytolysis, coagulative necrosis, vacuolization, perinuclear halo, and lymphocyte encroachment onto the myocyte. Sometimes myocyte injury is appreciated as an area with heavy lymphocytic inflammation where there has been myocyte dropout (empty spaces where myocytes should be) or "bits of myocytes" from the destructive process.