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Skin is part of the integumentary system and considered to be the largest organ of the human body. There are three main layers of skin: the epidermis, the dermis, and the hypodermis (subcutaneous fat). The focus of this topic is on the epidermal and dermal layers of skin. Skin appendages such as sweat glands, hair follicles,  and sebaceous glands are reviewed in-depth elsewhere.[1]

Melanocytes are neural crest-derived cells that sit in the stratum basalis of skin and hair follicles. These cells are responsible for the production of melanin from tyrosine. Melanin is a pigment that serves to protect DNA from UV radiation.  Different skin tones are due to differences in the amount of melanin, size, and density once transferred from melanocytes to keratinocytes via melanosomes, rather than the number of melanocytes. Destruction and unregulated proliferation of melanocytes lead to various pathologies such as vitiligo and melanoma.[6] Langerhans cells are specialized dendritic cells found predominantly in the skin. Much like macrophages, these cells are derived from bone marrow monocytes and are capable of antigen presentation.[7] Malignant proliferation of these cells are very rare and lead to a variety of diseases such grouped as Langerhans cell histiocytoses. Parakeratosis refers to corneocytes in the stratum corneum with retained nuclei. Though normal is some parts of the skin, this is abnormal is most. Parakeratosis is a sign of increased cell turnover and can present in disorders such as psoriasis.[8] Hyperkeratosis refers to the thickening of the stratum corneum due to an abnormal increase in keratin. It is often present in a condition like eczema, warts, and corns.