Skin immunity: understand and strengthen it with skincare ingredients

  • June 13, 2023
  • 6 minutes of read time

Skin immunity: understand and strengthen it with skincare ingredients

Face care

The COVID-19 pandemic has fundamentally changed consumers' attitudes towards health and wellness. In particular, it puts immunity in the spotlight like never before.

We all know that skin is the body’s largest organ and the first line of defense from external aggressors. So, with no surprise, the term “skinmunity” (i.e. skin immunity) has come up during the pandemic. It’s true that immune claims in facial care launches are still very niche, accounting for 1.3% of all facial care launches(1). At Seppic, we think they have the chance to grow in beauty, especially if immunity-focused beauty innovations bring substantiation and explanation proven by science. 

Today, you may have noticed that most cosmetic products refer to an enhancement of the skin's protective barrier to block external agents. However, skin is a complex organ and its biomolecules, structure, physico-chemical properties, immune and non-immune cells all act together to form the skin's innate immune system. Ultimately, the correct operation and balance of these elements guarantee the skin homeostasis. 


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What is skin immunity ?

One of the main functions of the skin is to protect the body against external aggressions, thanks to its structure, which supports its physical barrier properties, its biomolecules and a complex network of non-immune and immune cells. This ensemble is called the skin immune system.

The cutaneous immune system defends the skin against various and varied attacks, the best known of which are infections and diseases. This immune response is defined as the set of body defense mechanisms that discriminate between “self” and “not self” and fight pathogens. There are two types of immune response: the innate immune response and the adaptive immune response.

The innate immune response is the first line of defense against the infectious agents and pathogens that surround us, and this in all multicellular organisms. It is set up immediately and is functional for 4 days. It involves different defense modules, constitutive modules such as the skin barrier and induced modules such as phagocytosis, with phagocytic cells, and the inflammatory response, with cytokines. 

The adaptive immune response is the second line of defense against infectious agents and it only exists in vertebrae. It is set after 4 days and is characterized by the participation of lymphatic cells (lymphocytes, leukocytes or white blood cells). These lymphocytes are of two types, B lymphocytes (LB) responsible for the humoral response, i.e. the production of antibodies, and T lymphocytes (LT), responsible for the cytotoxic response, that is to say the destruction of the target by lysis.

Langerhans cells (LCs), constitutively present in the epidermis where they form a dense network, play the role of sentinels of the cutaneous immune system, directing it towards an innate or adaptive response.

Cells skin immunity Immune and non-immune skin cells

The innate immune response and the homeostasis of langerhans cells are the main targets to address for cosmetic purposes.

How to strengthen skin immunity with skincare ingredients ?

So, as an introduction, we said that the skin's main function is to ensure an efficient protection to external aggressors and stresses. However, skin is also a complex organ and to do so, different systems act in a coordinated way to establish the first line of defense of the skin

Let’s understand how these systems play a key role in skin immunity and how skincare ingredients can improve it.

Skin immunity mechanisms Mechanisms to target for skin immunity reinforcement

Skin immunity through the mechanical barrier

In normal conditions, the skin is impermeable to external agents. This role of mechanical barrier is guaranteed by the particular structure of the stratum corneum, also called the brick & mortar structure. 

In this structure, the corneocyte content, composed of keratins and proteins of terminal differentiation, ensures the rigidity of the epidermis. The epidermal lipids bind cells together and ensure the impermeability of the structure. Intercellular junctions strengthen cohesion of the upper layers of the epidermis. This structure prevents exogenous substance entrance and water loss.

It’s a well-known barrier that cosmetics usually address to reinforce the skin's defenses. But this is not the only one. 

Skin immunity through the “invisible” ecosystem

Moreover, the microbiota and its good balance are also crucial to skin’s barrier function. Keratinocytes are in constant contact with the external environment, including microbes, either commensal or pathogenic. In particular, commensal bacteria occupy accessible areas on skin and thus avoid the implantation of pathogenic bacteria, through the reinforcement of the immune barrier or by secreting antibacterial-like substances. In addition, commensal bacteria also stimulate the production of antimicrobial peptides by keratinocytes, stimulating the host’s immune defenses; some authors also mention that bacteria (such as Cutibacterium acnes) secrete fatty acids that help maintain the low pH of the skin, which also inhibits certain pathogens.

Skin immunity through the biochemical system

These antimicrobial peptides are secreted by keratinocytes and are part of the “chemical barrier” of the skin. Antimicrobial peptides, AMPs, are composed of molecules, such as defensins (HDB2), cathelicidins (LL-37) or also dermicidin, etc, fighting against external aggressors. Indirectly, these molecules also modulate immune response, by activating the immune cells, playing a role in the differentiation of monocytes in Langerhans cells and trigger keratinocyte migration and proliferation.

Moreover, the skin surface pH is usually at 5.4-5.9 and creates an inhospitable environment for pathogens. This pH is maintained thanks to the degradation of filaggrin in histidine and then trans-uronic acid and to acidic electrolytes and lactic acid of sweat glands. 

Skin immunity through the immune system

The skin immune system is ensured in part by the immune cells such as Langerhans cells.

These cells are dendritic cells and represent 3 to 5% of all cells in the epidermis. Under normal conditions, LCs are located in the epidermis and form a network around keratinocytes, thus participating in tissue homeostasis by secreting growth factors necessary for the survival of keratinocytes, fibroblasts and endothelial cells. They also express a CD39 transmembrane protein with the ability to hydrolyze extracellular ATP, which can be recognized as DAMPs (molecular patterns associated with damage) and induce an inflammatory reaction. Finally, Langerhans cells maintain optimal tissue function. As cleaners they phagocytize debris and apoptotic cells, thus promoting tolerance.

Indeed - and this is their key role - LCs determine the immune response by interpreting the microenvironmental context in which they face foreign substances. LCs are capable of promoting tolerance under conditions of homeostasis or initiating an innate or even an adaptive immune response in the event of an external attack.

The critical importance of LCs ability to discriminate between signals that indicate danger and those that are not threatening is reflected in immunological tolerance. This tolerance prevents adaptive immune responses against commonly encountered environmental substances. Conversely, when a danger signal is identified, Langerhans cells are able to migrate to the lymph nodes and present the antigen to cytotoxic T lymphocytes and thus initiate an adaptive immune response.

In addition, we can mention Calcitonin gene-related peptide (CGRP), which has key regulatory functions on immune and inflammatory processes within the skin. CGRP released by nerves is intimately associated with epidermal LCs and CGRP has profound regulatory effects on Langerhans cell antigen-presenting capability, ultimately modulating the immune response.

Skin immunity and skincare ingredients

Then, cosmetic products can help to preserve skin innate immunity in the epidermis by acting on different mechanisms and targets such as: 

  • reinforcing the “brick & mortar” properties of the stratum corneum
  • preserving the balance of antimicrobial peptide synthesis
  • balancing the microbiota
  • maintaining the skin acidic mantle
  • decreasing the neuronal suractivation 
  • protecting the homeostasis of cutaneous sentinel cells

At Seppic, with strong expertise in skin biology, we aim to share our knowledge and support brands in creating preserved immunity focused beauty products with a specific selection of skincare  ingredients.

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Well, and what are the ultimate benefits of an improved skin immunity ? 

Actually, a stronger skin immune well-being has a number of happy side effects. Skin is healthier, more resilient and comfortable.

To remember
  1. To fight against external aggressions, the skin has two lines of defense: the innate and adaptative immune responses.
  2. The innate immune response, which is fast but non specific to an antigen, is the main target to address for cosmetic purposes.
  3. In both cases, at skin level, Langerhans cells are indispensable as skin sentinel, orienting the answer toward an innate or adaptive process.