We have discovered that administering anti-ceramide antibody treats and prevents an array of diseases mediated by cytolytic T lymphocyte (CTLs)-induced killing and by damage to endothelial microvasculture, including radiation-induced GI syndrome, Graft vs. Host diseases, inflammatory diseases and autoimmune diseases. We have also discovered new anti-ceramide monoclonal antibodies, that have therapeutic use preferably in humanized form to treat or prevent these diseases.