SUMMARY Dendritic cells are antigen-presenting cells found in almost every type of tissue, including lymphatic tissue, blood and skin. In the intes- tinal tract, these cells are likely to play a pivotal role in the initiation and regulation of immune responses. Our earlier study of the human colon and ileum revealed the presence of zinc iodide- osmium-positive dendritic cells. In the present study we demonstrate the presence of ZIO-pos- itive dendritic cells in the human appendix. ZIO-positive cells were seen in the region of crypts of Lieberkühn as well as in the surface epithelium. The cells showed a single long process directed towards the lumen. They were long, slender, and triangular. In the region of the lymphoid follicle, two different types of dendritic cells were noted. The follicular den- dritic cells present in the germinal center were few in number, larger in size and with thick dendritic processes. However, in the mantle zone typical dendritic cells were seen. They were smaller in size and had many thin process- es. The distribution of dendritic cells in the human appendix confirms the role of the appendix in the immune response. Key words: Follicular dendritic cells – ZIO – Human appendix – Crypts of Lieberkühn I NTRODUCTION Dendritic cells (DCs) are potent immunos- timulatory cells (Steinman, 1991). They can take up and present both intestinally and oral- ly administered antigen to naïve T cells (Liu et al., 1991; Liu and MacPherson, 1993). Den- dritic cells are not only critical for the induc- tion of primary immune responses, but may also be important for the induction of immunological tolerance as well as for the reg- ulation of the type of T cell-mediated immune response (Banchereau et al., 2000). DCs express several Ca 2+ -dependent (C- type) endocytic lectins, exemplified by DEC 205 (Jiang et al., 1995), the mannose receptor (Sallusto et al., 1995) or Langerin (Valladeau et al., 2000). Although several markers have been used, Zinc-Iodide-Osmium (ZIO) has been extensively employed to identify the presence of dendritic cells in human and ani- mal epithelia and subepithelial tissue (Crock- er and Hopkins, 1984; Dagdeviren et al., 1994; Breathnach and Goodwin, 1965; Niebauer et al., 1969; Rodriguez and Caorsi, 1978; Hart and Fabre, 1981; Sertl et al., 1986; Prickett et al., 1988; Steinman, 1991). Gut-associated lymphoid tissue (GALT) is the largest lymphoid organ in the body (Chan- dran et al., 2003). Dendritic cells are present in GALT, such as Peyer’s patches, and are scat- Eur J Anat, 10 (1): 15-20 (2006) 15 Distr ibution of zinc iodide-osmium positive dendr itic cells in the human appendix S. Rabi, I. Indrasingh and S. Koshy Department of Anatomy, Christian Medical College, Vellore, India Correspondence to: Dr. J. Suganthy. Department of Anatomy, Christian Medical College, Vellore 632 002, India. Phone: 0091-0416-2284245/ 2284387. E-mail: suganthyrabi@cmcvellore.ac.in Submitted: September 29, 2005 Accepted: March 10, 2006