How to recognise the different types of redness?

The acne appears most often on oily skin, owing to an overproduction of sebum by the sebaceous glands. Excess sebum clogs the pores and creates an environment conducive to the proliferation of Cutibacterium acnes, a bacterium naturally present on the skin but which becomes pathogenic when it multiplies. By feeding on the triglycerides contained in sebum, C. acnes releases pro-inflammatory fatty acids that irritate the wall of the hair follicles. This mechanism then triggers a local inflammation, explaining the appearance of redness around the spots.

When inflammatory acne develops, the body activates an immune response. Neutrophils and macrophages are then recruited to the affected area, while chemical mediators such as interleukins and prostaglandins are released into the skin. These pro-inflammatory molecules lead to a vasodilatation of the blood capillaries, that is an increase in their diameter to supply more blood, oxygen and nutrients to the tissues. This phenomenon amplifies the redness, making blemishes more noticeable, particularly on fair skin.

Even after blemishes have cleared, some redness may persist: this is referred to as post-inflammatory erythema (PIE). These flat pink to reddish spots appear at the sites of former lesions. They result from residual dilation of capillaries rather than an excess of melanin, unlike post-inflammatory hyperpigmentation, which more often affects darker skin types. Although the redness associated with previous imperfections is not classified as scarring, it can still undermine skin tone uniformity and impact self-esteem.

Post-inflammatory erythemas tend to fade on their own, but their disappearance can be prolonged, ranging from a few months to several years, depending on skin phototype, the severity of the initial inflammation and sun exposure habits. Indeed, UV radiation stimulates free radical production and slows skin regeneration, which tends to prolong the persistence of redness. This is one of the reasons why daily sun protection is essential, particularly for individuals prone to redness.

Redness resulting from the dilation of blood capillaries mainly presents as couperose or rosacea, chronic conditions primarily affecting the central region of the face (cheeks, nose, forehead and chin). It should be noted that couperose corresponds to the vascular form of rosacea, the most common, where the dermal capillaries dilate excessively, resulting in visible redness and occasionally sensations of warmth.

Episodes of redness are particularly common in sensitive skin, where the skin barrier is impaired and sensory nerve fibres are highly reactive. They may be temporary, subsiding within minutes or hours (referred to as erythema), or persist for days, even weeks (known as erythrosis). Although more frequent in fair phototypes, all skin tones can be affected by rosacea. The exact causes of rosacea remain only partially understood, but several biological and environmental factors seem to favour the dilation of blood vessels:

• Genetics : Up to 30% of people with rosacea have a family history, suggesting a hereditary component. Certain receptors involved in inflammatory responses may be more prevalent in these individuals.

• Colonisation by the Demodex : When they multiply, these tiny parasites naturally present on the skin can secrete proteases that activate epidermal PAR-2 receptors. This activation leads to the release of pro-inflammatory cytokines, such as TNF-α and IL-1, which are responsible for inflammation and redness.

• Heat and the consumption of spicy foods : Stimulation of TRPV1 thermosensitive receptors by heat or by compounds such as capsaicin, found in spicy foods, induces local vasodilation and exacerbates hot flushes and facial flushing.

• Stress : Stress can activate TRPA1 and TRPV1 receptors, triggering the release of vasodilatory neuropeptides such as CGRP and PACAP. These molecules favour capillary dilation and exacerbate erythema. Cortisol may also play a role in this process, although the exact mechanism remains unknown.

• UVB exposure : UVB rays penetrate the epidermis and stimulate TRPV4 receptors, contributing to the deterioration of skin structure and vasodilation, thereby accentuating redness.

Note : The rosacea predominantly affects women and most often occurs in their thirties.

Redness may also occur following exposure of the skin to an allergen or irritant, triggering what is known as contact eczema, or contact dermatitis. Unlike atopic eczema, this condition is not related to a genetic predisposition to allergies. It is an acquired inflammatory reaction of the skin, which can manifest even in individuals without any atopic history. The allergens responsible for this irritation-induced redness can originate from a variety of sources: clothing, adhesives, nickel-containing jewellery, cosmetics, medications, cement, paints, pesticides...

Contact dermatitis develops in two phases. The first is the sensitisation phase, during which the allergen penetrates the skin and binds to skin proteins to form an antigenic complex. T lymphocytes of the immune system recognise this complex and commit the allergen to memory, without immediately producing any visible symptoms. This phase can last from several days to several weeks.

Upon re-exposure to the allergen, activated lymphocytes release lymphokines and other inflammatory mediators. It is at this point that the so-called elicitation phase. This response leads to characteristic symptoms: erythema, pruritus, oedema, vesicles, desquamation, skin thickening… Lesions typically appear at the contact site, but may, in severe cases, extend over large areas of skin.

Irritation-induced redness resolves within one to two weeks following removal of the causative allergen.

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• HAKOZAKI T. & al. Skin inflammatory signatures, as measured by disordered spatial redness patterns, predict current and future skin ageing attributes. Experimental Dermatology (2024).