tnf-α and become proinflammatory effector cells. dendritic cells dcs present less than 10% of lamina propria apc and typically do not reside in lower layers. inherent production of retinoic acid and tgf-β (typical for gut-associated dcs) induces expression of gut-homing molecules and favor iga switch during maturation of b cells in folicules. dcs also direct treg and conventional iels to receive their final phenotype of mature effector cells in intestine. other animals the adaptive immunity, mediated by antibodies and t cells, is only found in vertebrates. whereas all of them have a gut-associated lymphoid tissue and the vast majority have a version of spleen and thymus, not all vertebrates show bone marrow, lymph nodes or germinal centers, what means that not all vertebrates can generate lymphocytes in bone marrow. this different distribution of the adaptive organs in the different groups of vertebrates suggests galt as the very first part of the adaptive immune system in vertebrates. it has been suggested that from this existing galt, and due to the pressure put by commensal bacteria in gut that coevolved with vertebrates, later specializations as thymus, spleen or lymph nodes appeared as part of the adaptive immune system. additional images lymphatics of colon. section of the human esophagus. transverse section of human vermiform process. x 20. section of mucous membrane of human rectum. x 60.