The Skin Immune System and Intradermal Delivery of Vaccines: A Review

The skin is considered as the largest organ in the body, functioning as a barrier providing protection from the outside environment, but also performing essential immune functions through a complex network of epidermal and dermal cells that interact with each other. The basic principle of the vaccines is to induce protection against pathogens by simulating its interaction with the immune system, allowing to generate a memory immune response. To achieve this protection, the interaction and binding of both innate and adaptive immune responses is required. Intradermal (ID) delivery of vaccines achieves direct injection of the antigen into the dermis, where the largest numbers of immune cells are found (macrophages, dendritic cells, Langerhans cells, B and T lymphocytes, and mast cells, among others). It is a novel route that elicits antibody responses equivalent to other routes of administration but at lower doses, a phenomenon known as "dose saving". This route also allows for better thermo-stability of the antigen, fewer booster immunizations and, as a consequence, increased adherence to the vaccination regimens with less burden on the medical personnel. There are currently several vaccines for the ID administration on the market, and several more under development; with good safety profiles and efficacy rates. In this article, we review the most important aspects of the immune system within the skin, the pathways by which vaccines are applied to the skin intradermally to produce an adequate immune response, and also their advantages and disadvantages.The skin has important immune machinery, thanks to which both innate and adaptive immune responses merge. This interaction allows for the basis of vaccination: development of memory responses to various antigens, providing protection for the future re-exposures.