Proposed schema of pathophysiology in aGVHD of colon in the context of steroid response. A typical SS focus would have immune cells related to classic CMI response, amenable to steroid (S) therapy. The epithelium would be leaky with overactive ion channels, having experienced LG damage with mainly apoptosis and the occasional crypt loss. The endothelium would show an inflammatory signature. Contrarily, an SR focus would have a neutrophil-rich immune milieu with stunted CMI response, possibly secondary to genotoxic damage to host-derived antigen-presenting cells. There would be HG pathology in colonic epithelium with extensive crypt loss, and the vessels would exhibit a near basal functioning phenotype with some neoangiogenic signatures.