Anatomy

The skin is the largest organ of the body. Life is not possible if skin is completely absent. The skin has many functions: immune responses to exogenous antigens, protection of the body from the elements, thermal regulation, and sensory perception.

There are two main layers of skin: the epidermis and the dermis, the latter lying upon the supporting subcutaneous tissue. The cells within the epidermis are: keratinocytes, melanocytes, Langherhans' cells, and Merkel cells. Keratinocytes mature and differentiate to form keratin which serves as the protective coating of the body. Melanocytes produce pigment (melanin). Langherhans' cells participate in the immune respone of the body by being involved in antigen processing. Merkel cells are sensory cells and other functions.

An important boundary between the epidermis and dermis is the basement membrane. Many proteins are present within the basement membrane (collagen, laminins, plectins, etc.). Hemidesmosomes and anchoring fibrils attach the epidermis to the basement membrane and dermis. Many diseases, especially autoimmune diseases, affect the basement membrane of the skin directly or indirectly.

The dermis contains fine blood filled capillaries, arterioles, venules, lymphatics, nerve endings and structures involved in sensory perception and thermoregulation. Fibroblasts produce collagen, adipocytes produce fat, and macrophages scavenge for intrinsic and extrinsic antigens and cellular debris. Elastic and reticular fibers are present within the dermis. Elastic fibers decrease as one ages with subsequent loss of skin tone.