Understanding the skin’s integrated defence systems.
By Jacinta Curnow, 2025 ABIA Sole Operator of the Year.
For many years, the skin barrier has been explained using the familiar “brick and mortar” model, with skin cells as the bricks and intercellular lipids as the mortar. While this analogy provides an excellent foundation for understanding barrier function, our knowledge of skin physiology has evolved considerably.
The skin barrier is far more than a physical wall. It is a sophisticated defence network made up of multiple systems that work together to protect against pathogens, regulate immune responses, minimise water loss and maintain homeostasis. When one component is compromised, the others are inevitably affected.
Barrier as an Integrated System
Each of these As skin therapists, understanding these integrated defence systems allows us to move beyond simply repairing the barrier and towards supporting optimal skin function.
1. The Chemical & Microbial Defence
The skin’s first line of defence begins before we even reach the corneocytes.
The acid mantle is a thin, slightly acidic film that sits on the skin’s surface, typically maintaining a pH between 4.5 and 5.5. This acidic environment supports healthy enzyme activity, assists in the formation of barrier lipids and discourages the growth of pathogenic microorganisms.
Living within this environment is the skin microbiome. A diverse ecosystem of bacteria, fungi, viruses and mites that work symbiotically with the skin. These microorganisms help regulate inflammation, educate the immune system and compete against harmful microbes.
The acid mantle and microbiome should not be viewed as separate entities. The acid mantle creates the environment, while the microbiome thrives within it. Disrupting one inevitably influences the other.
2. The Physical Defence: Corneocytes
Corneocytes form the outermost layer of the epidermis and remain one of the most recognised components of barrier function. As the skin’s primary interface with the external environment, they provide the first physical point of contact between our body and the outside world.
Corneocytes are the body’s first point of contact with the outside world. Although these flattened cells no longer contain a nucleus, they provide remarkable protection by reducing transepidermal water loss (TEWL), resisting mechanical injury and shielding the skin from environmental stressors.
Healthy skin isn’t simply about producing corneocytes, it’s about maintaining their organisation, communication to the dermis, cohesion and integrity.
3. The Lipid Defence: Lamellar Lipids
If corneocytes are the bricks, then lamellar lipids are the mortar holding everything together.
These organised lipid bilayers, primarily composed of ceramides, cholesterol and free fatty acids, fill the spaces between corneocytes to create a flexible, waterproof yet breathable barrier.
When this lipid matrix becomes depleted, the barrier becomes increasingly permeable, allowing greater water loss while making the skin more vulnerable to irritation, inflammation and environmental insult.
Barrier repair therefore extends beyond hydration alone. Restoring lipid composition is fundamental to long-term skin resilience.
4. The Cellular Defence: Keratinocytes
Keratinocytes account for approximately 90% of the epidermis, yet their role extends far beyond structural support.
These living cells continually communicate with surrounding tissues by releasing cytokines, producing antimicrobial peptides and coordinating wound healing responses. They act as active participants in maintaining skin homeostasis, constantly responding to injury, infection and environmental change.
Rather than passive building blocks, keratinocytes are intelligent communicators within the barrier network.
5. The Immune Defence: Langerhans Cells
Embedded throughout the epidermis are specialised immune cells known as Langerhans cells.
Often referred to as the skin’s immune sentinels, they continually survey the epidermis, identifying foreign antigens and initiating immune responses when required. Their role is not only to defend against pathogens but also to maintain immune tolerance and regulate inflammation.
When barrier integrity is compromised, Langerhans cells become increasingly activated, highlighting the close relationship between barrier dysfunction and inflammatory skin conditions.
6. The Photoprotective Defence: Melanocytes
Melanocytes are often associated solely with pigmentation and sometimes written as the villain in many skin stories, yet their protective role extends far beyond determining skin colour.
By producing melanin, these specialised cells absorb and disperse ultraviolet radiation, helping protect cellular DNA from oxidative damage and photoaging.
Pigmentation is therefore not simply a cosmetic response, it is an adaptive defence mechanism designed to protect the skin from environmental stress.
Seeing the Barrier as an Integrated System
Each of these defence systems performs a unique role, but none functions independently. The acid mantle supports the microbiome. The microbiome communicates with immune cells. Keratinocytes signal inflammation. Lipids maintain structural integrity, while melanocytes protect against ultraviolet damage.
The skin barrier is not a single structure, it is an intelligent, interconnected defence network.
As our understanding of skin physiology continues to evolve, so too should our treatment philosophy. Rather than asking, “How do I repair the skin barrier?” perhaps the better question is, “Which part of the skin’s defence network needs support?”
By recognising the barrier as more than bricks and mortar, we can deliver more thoughtful, individualised treatments that support not only the skin’s structure, but it’s remarkable ability to protect, communicate and heal.
Jacinta Curnow is a Holistic Clinical Skin Therapist, Corneotherapist and Health Science (Naturopathy) student. Passionate about bridging skin physiology with whole-body health, she educates therapists on barrier function, the gut–skin axis and evidence-informed approaches to long-term skin health.
Jacinta Curnow
Holistic Skin Therapist & Educator
Bachelor of Health Science (Naturopathy- in progress)
@ jacintacurnowskin
www.jacintacurnowskin.com
