1. What is the skin barrier?
The skin is our first defense against the external environment. This complex barrier defends the body against various attacks: chemical attacks, by toxic substances or allergens, physical attacks which can be mechanical or for example linked to UV radiation (responsible for skin aging) as well as microbiological attacks, that is to say by bacteria, viruses or fungi.
The structure and properties of our skin such as its thickness, transcutaneous water loss or sebum secretion vary from one area of the body to another. Among its layers, the stratum corneum or stratum corneum which is the outermost layer of the skin, plays a fundamental role.
2. What are its roles?
Our skin barrier assumes four main functions : physical, chemical, microbiological and immunological protection. These four functions interact to ensure stability, hydration, avoid imbalances in the skin microbiome and reduce inflammatory reactions of the skin.
Physical protection
This protection is mainly provided by the first layer of the skin called stratum corneum. This ensures the tightness of the skin and limits transcutaneous water loss. It is composed of dead cells (corneocytes) linked together by specialized proteins (fillagrin, corneodesmosomes, tight junctions) and lipids (fatty acids, cholesterol, ceramides) which form an intercellular cement. This system makes the skin waterproof and less permeable to pathogenic microorganisms (bacteria, viruses or fungi).
Chemical protection
The chemical protection of the skin is composed in particular of lipids (such as sebum) and natural hydration factors, which make it possible to maintain an acidic pH between 4 and 6. This acidity slows the development of harmful bacteria while promoting enzymatic processes in the skin such as cornification (cell renewal mechanism), the elimination of dead cells and the synthesis of lipids. Keratinocytes also secrete antimicrobial peptides (defensins, cathelicidins) which prevent the growth of unwanted bacteria and germs and activate the skin's immune defense.
Microbiological protection
Our skin is home to micro-organisms (bacteria, fungi, viruses) which reside and live in harmony with it. Their interaction with the skin barrier forms an additional defense against attacks. From birth, the skin is colonized by microorganisms which evolve over time, with notable changes during puberty under the effect of hormones and increased sebum production. This microbiome actively participates in the protection of our body by preventing harmful micro-organisms from developing thanks in particular to maintaining the acidic pH. It also stimulates the skin's immune system.
Immunological protection
Finally, the skin has a built-in immune system. Immune cells, interacting with the microbiome, detect external threats and activate an immune response. This system is composed of resident cells (Langherans cells, dendritic cells) which provide innate (rapid reaction to intruders) and adaptive (more specific response) immunity. Thus, the skin reacts and adapts according to threats, while maintaining its balance.
Key points:
The skin barrier plays an essential role in protecting us from external aggressions and maintaining water in our body. It is based on four main roles: a physical barrier (stratum corneum and lipids), chemical (acidic pH and antimicrobial peptides), microbiological (skin microbiome), and immune (Langerhans cells and immune cells). Together, these roles make our skin a remarkable defense organ, and maintaining its integrity is essential for optimal skin health.
