Skin

The skin is the largest organ of the human body and serves not only as a mechanical and chemical barrier but also as an active immune organ. It plays an essential role in both innate and adaptive immune responses, functioning as a peripheral immune surveillance site comparable to secondary lymphoid organs.

Structure: The skin is composed of three main layers:

• Epidermis: Derived from the ectoderm, it is a stratified squamous epithelial tissue primarily composed of keratinocytes, along with melanocytes, Langerhans cells, and Merkel cells.

• Dermis: Derived from the mesoderm, it consists of connective tissue, blood vessels, lymphatic vessels, and skin-associated glands (sebaceous and sweat glands).

• Basement membrane: A specialized structure that separates the epidermis from the dermis, maintaining structural integrity and regulating selective permeability between the two layers.

Functions:

• Barrier function: Provides mechanical protection and waterproofing through keratinized epithelial layers.

• Innate defense: Sweat and sebaceous glands secrete antimicrobial compounds that contribute to the skin’s first line of defense.

• Immune function: Keratinocytes express MHC class II molecules, enabling limited antigen presentation. Langerhans (dendritic) cells capture and process antigens, migrating to regional lymph nodes to activate T lymphocytes.

Resident T lymphocytes, including those with semi-innate receptors, continuously patrol the epidermis and dermis, providing rapid immune responses against invading pathogens.

Beyond its role as a physical barrier, the skin functions as an immunologically active tissue that integrates structural, innate, and adaptive defense mechanisms. It acts as a peripheral, secondary lymphoid-like organ involved in continuous immune surveillance and host protection.