Skin: what is its function?

The skin constitutes the first line of defense of the body in confronting the external environment. Its barrier function relies on a multilayered organization of the skin tissue, where each component helps limit the penetration of foreign substances and prevents dehydration. The stratum corneum, composed of corneocytes embedded in a lipid matrix (ceramides, cholesterol, free fatty acids), acts as a semi-permeable physical barrier. This “brick and mortar” architecture minimizes transepidermal water loss and blocks the entry of pathogens or irritants.

This mechanical role is complemented by a chemical barrier function. The hydrolipid film, present on the surface of the epidermis, is slightly acidic (pH between 4.5 and 5.5), which limits microbial proliferation and stabilizes the skin microbiome. The sebaceous glands and sweat glands actively contribute by releasing fatty acids, salts, and antimicrobial peptides, such as defensins or cathelicidin, which can act against pathogenic bacteria. This chemical environment of the skin helps to prevent infections.

Finally, the skin has an integrated immunological barrier, composed of immunocompetent cells present even in the superficial layers of the epidermis. The keratinocytes themselves contribute to the immune response by releasing proinflammatory cytokines when the skin is challenged. Furthermore, Langerhans cells, macrophages, and T lymphocytes ensure the recognition and elimination of foreign bodies.

The skin is a true immunological interface, hosting a complex network of cells and mediators tasked with defending against pathogens while maintaining balance with the cutaneous microbiome. Keratinocytes and sebocytes actively participate in innate immunity through their pattern recognition receptors, such as Toll-like receptors (TLRs) and NOD-like receptors (NLRs). These sensors detect microbial patterns (lipopolysaccharides, peptidoglycans, bacterial or viral nucleic acids) and trigger the activation of T lymphocytes, cells of the immune system capable of eliminating pathogens and tumor cells.

Skin immunity also relies on the production of antimicrobial peptides and lipids. Human β-defensins (hBD1-3) and the cathelicidin LL-37 modulate the immune response by recruiting macrophages, mast cells, and dendritic cells. Other epithelial proteins, such as Small Proline Rich Proteins (SPRR), help limit bacterial colonization, while sebaceous lipids, like sapienic acid and linoleic acid, exhibit bactericidal activity by disrupting microbial membranes. These various mechanisms enable the skin to maintain a balanced microbial flora and prevent the proliferation of pathogens.

Finally, the skin harbors a vast reservoir of resident immune cells, including Langerhans cells, the epidermal sentinels that capture antigens and activate memory T cells. The dermis also harbors dendritic cells, macrophages, mast cells, and regulatory T cells (Treg) that participate in the limitation of inflammation. Resident memory T cells (Trm), estimated at over 20 billion in human skin, ensure rapid local protection upon re-exposure to a pathogen.

The skin also plays a key role in sensory perception. Thanks to its high density of nerve receptors, it detects a wide variety of stimuli, whether tactile, thermal, painful, or vibratory, and transmits these signals to the brain’s sensory cortex. This rapid communication allows the body to respond instantly, as in a withdrawal reflex from a heat source. Certain areas, such as the fingertips or lips, have a particularly high concentration of nerve endings, giving them exceptional tactile sensitivity.

Skin sensory receptors are divided into encapsulated endings, such as Meissner, Pacinian, or Ruffini corpuscles, and unencapsulated endings. The former, protected by a connective tissue capsule, detect pressure, vibrations, and skin stretch. The free nerve endings, which lack a sheath, mediate the perception of pain—also called nociception—and temperature—also called thermoception—especially in glabrous areas like the palms of the hands and the soles of the feet.

The skin also plays a role in the body's thermal regulation, maintaining an internal temperature of approximately 37°C despite environmental fluctuations. This is possible thanks to the dense vascular network of the skin. When it is hot, the cutaneous blood vessels dilate, increasing blood flow to the skin’s surface so that heat can dissipate. Sweating further enhances this process, as the evaporation of sweat at the epidermal surface cools the skin. Conversely, when temperatures drop, the blood vessels constrict to limit surface blood flow and preserve internal heat, while the arrector pili muscles contract to produce goosebumps.

Beyond these reflex mechanisms, skin thermoregulation is controlled by the autonomic nervous system and by hormonal mediators, such as epinephrine or norepinephrine. Cutaneous thermoreceptors, sensitive to local temperature variations, transmit signals to the hypothalamus, the control center for body temperature. The skin thus acts as both a sensor and an effector: it detects thermal changes and implements the physiological responses necessary to maintain homeostasis.

The skin can also be considered a peripheral neuroendocrine organ capable of sensing, producing, and responding to neural and hormonal signals. This function relies on a dense network of sensory and autonomic (sympathetic and parasympathetic) nerve fibers, which closely interact with skin cells and the immune system. The nerves release various neurotransmitters, such as acetylcholine (ACh), norepinephrine, substance P, or the calcitonin gene–related peptide (CGRP), which act on specific receptors on the surface of keratinocytes, fibroblasts, and immune cells. At the same time, skin cells express hormonal receptors similar to those found in the brain and endocrine glands, allowing them to respond to systemic hormones such as cortisol or epinephrine.

These interactions modulate various functions, such as cell growth and wound healing, vasodilation, as well as the inflammatory response.

The sympathetic nervous system plays a major role in regulating microcirculation and stress responses via the release of norepinephrine, which controls vasoconstriction and sweat production. Conversely, the parasympathetic system, via acetylcholine promotes cellular regeneration and relaxation processes, contributing to the maintenance of skin homeostasis. These systems are modulated by the hypothalamic–pituitary–adrenal axis, which, in response to stress, induces cortisol production. Cortisol exerts a short-term anti-inflammatory action but can impair barrier function and skin repair when produced chronically. Thus, the skin is in constant dialogue with the central nervous and endocrine systems, forming a bidirectional network that allows for adapting cutaneous responses to stimuli external (UV, heat, microbiota) and internal (stress, emotions).

The skin is also well-known for its metabolic role, particularly in ensuring the synthesis of vitamin D. Under the influence of UVB rays, the 7-dehydrocholesterol present in the epidermis is converted into previtamin D3, which is then transformed into active vitamin D3 by the liver and kidneys. The latter participates in the regulation of calcium and phosphorus, and thus in bone health, but also influences other metabolic functions, such as modulation of the immune response and support of muscle function. The skin’s capacity to produce vitamin D depends on several factors, including age, the skin phototype, sun exposure and the integrity of the skin barrier.

Note : Contrary to a widespread misconception, the sunscreen does not cause vitamin D deficiency.

The skin is one of the first features others notice in us, making it a true mirror of our health. It not only serves as a physical barrier against external aggressions, but it also reflects our internal states, whether physiological or psychological. Visible skin conditions, such as acne, eczema, psoriasis, as well as temporary changes related to stress, fatigue, or hormonal imbalances, can reveal disturbances within the body, whether immune, endocrine, or psychological in nature. These manifestations have a direct impact on how a person is socially perceived: they can alter daily interactions, influence self-confidence, and affect the subjective perception of one’s appearance.

The skin thus plays a central role, both biologically and socially.

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