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Autoimmune Diseases of the Skin To understand the skin antigens targeted by autoimmune disease it is necessary to have clear the normal understanding of the structure and function of the skin.  Figure 1 The skin is composed of three layers:
The EPIDERMIS is a stratified squamous epithelium composed mainly of keratinocytes, which migrate constantly from the deepest layer (the basal layer) to the most superficial (the corneum layer) Figure 2.  Keratinocytes are attached to each other via desmosomes, and to the basement membrane via hemidesmosomes. Desmosomes are specialized, symmetric junctions composed of a cytoplasmic and an intercellular region, Figure 3 (LINK). The cytoplasmic region is a dense plaque (called attachment plaque) and contains desmoplakins (DP) and plakoglobins (PG) proteins, which remain intracellular. The intercellular region consists of desmocollins (DC) and desmogleins (DG) proteins, which originate in the attachment plaque but then protrude into the intercellular spaces between two keratinocytes. The hemidesmosome, like the desmosome, consists of a dense plaque and an associated bundle of keratin filaments; it lacks, however, a mirror image of itself at the junction and has a different protein composition than the desmosome.  The hemidesmosome contains in fact two proteins that are not present in the desmosome: the bullous pemphigoid antigen 230 (BP230), an intracellular component of the hemidesmosomal plaque, and the bullous pemphigoid antigen 180 (BP180), that crosses the plasma cell membrane and extends extracellulary into the lamina lucida. The basement membrane separates the epidermis from the underlying dermis, and has two components: a superficial, electron-clear layer, called the lamina lucida, and a deep, granular, electron-dense layer, called lamina densa (or basal lamina). The lamina lucida contains, in addition to the bullous pemphigoid antigen 180, a large cruciate glycoprotein, called laminin. The lamina densa is made up mainly of type IV collagen, organized in a netlike structure, and heparan sulfate proteoglycan, that may serve as a sieve regulating the molecular traffic through this area. Anchoring fibrils, made up of type VII collagen, attach the lamina densa to the underlying dermis. In addition to keratinocytes, the epidermis contains other cell types, such as melanocytes. Melanocytes are located in the basal layer and appear as round cells with pale cytoplasm. They produce the brown pigment melanin, which is mainly responsible for skin coloration. The amount of pigment varies from race to race, but racially white-skinned people can increase the amount of melanin synthesized and deposited by increasing the exposure of the skin to ultraviolet light. |
