Biology of the crypts of Lieberkühn. Paneth cells generate defensins (red crosses) that are involved in controlling the growth of bacterial organisms in the lumen of the crypts of Lieberkühn (bacteria shown as green ovals). Production of defensins is partly mediated by the response of Paneth cells to bacterial products present in the lumen of the crypt (bacterial products shown as black circles). T-cell immune response (labeled) leads to loss of Paneth cells, inhibiting the generation of defensins, and bacterial translocation across the mucosa and into the systemic circulation. In addition, Paneth cell destruction leads to the release of RegIIIα (light blue triangle), which serves as a marker for the presence of GvHD. Professional illustration by Kenneth X. Probst.