INTRODUCTION

The skin is the largest human organ, and its surface (measuring about 2 square meters) protects the body from invading organisms, toxins, and viruses. Loss of the protective epidermal barrier (secondary to burns or disease states) results in an increased risk of infection from a variety of organisms.

Normal skin may be divided into three major parts: epidermis, dermis, and hypodermis. The epidermis is a stratified, squamous epithelia composed mainly of keratinocytes that differentiate to a physical barrier (the stratum corneum). The dermis is composed largely of collagen and elastin fibers synthesized by dermal fibroblasts. The dermis also contains a rich vascular supply, nerves, and various appendages such as hair follicles, eccrine glands, and apocrine glands. The hypodermis contains subcutaneous adipose tissue with associated vascular and neural elements and contributes the largest bulk of the cutaneous organ.

The skin is also an important immunological organ and is capable of mediating or initiating both innate and acquired immune responses. Keratinocytes synthesize a range of proteins such as defensins that directly kill bacteria. Activated keratinocytes can also rapidly recruit neutrophils and other innate immune cells through release of a large number of mediators such as S100 proteins, chemokines, cytokines, and lipidderived molecules. Keratinocytes can also activate acquired immunity through synthesis of heat shock proteins that activate dendritic cells (DCs) in the skin.

Normal skin contains at least two resident populations of antigen-presenting DCs Langerhans cells and dermal DCs Langerhans cells are randomly distributed throughout the living cell layers of the epidermis and constitute about 1 percent of all epidermal cells.