The skin contains high concentrations of antigen-presenting cells, and is thus a site capable of inducing potent immune responses. The skin is composed of multiple layers, each with characteristic resident and transient immune cell subsets. Outermost is the thin layer of the epidermis (0.05-0.2 mm), which is primarily made up of epithelial cells as well as Langerhans cells, melanocytes, and Merkel cells. Beneath the epidermis, the dermis is a thicker layer (1.5-3 mm) consisting of a network of collagen fibers. Cells of the adaptive and innate system reside in or circulate through the dermis, including macrophages, mast cells, Langerhans cells, and dermal dendritic cells. Antigenpresenting cells in the skin perform an essential role in processing incoming antigens, resulting in immune system activation or immune tolerance of self or harmless antigens (Nicolas and Guy 2008) . For these reasons, it is possible that delivery of vaccines to the epidermis or dermis may result in superior immune responses compared to other anatomical sites (Glenn and Kenney 2006; Lambert and Laurent 2008; Nicolas and Guy 2008) . Alternatively, an equivalent immune response could be stimulated by delivery of a smaller quantity of vaccine antigen to the skin. Either of these mechanisms could be beneficial for developing vaccines against new disease targets, improving immune responses in hard-to-treat groups, or lowering the cost of vaccine antigens, and may be particularly valuable for improving access to vaccines in low-resource settings.