Dendritic cells (DC) are professional antigen presenting cells characterized morphologically by the extension of numerous dendrites, phenotypically by the expression of relatively large amounts of MHC class II molecules and costimulatory molecules, and functionally by their potent capacity to activate immunologically naive T cells. Members of this family reside not only in lymphoid tissues (e.g., spleen, lymph node, and thymus), but also in epithelial tissues at the environmental interface (e.g., skin). Therefore, external antigens that penetrate into bodies can be readily presented by DC to the immune system.