The Protective Acid Mantle
For over 100 years the function of the protective acid mantle (i) has been under discussion. Initially, the focus was on indirect microbiological defence and direct protection against alkaline noxae (i). This classical knowledge of the importance of the pH (i) for the skin, however, has since been expanded by more recent biochemical and molecular biological studies.
More recent research findings prove that the acid pH (i) of the horny layer plays an essential role in the formation and structure of the epidermal lipids and with them the permeability barrier (i). These studies showed that an acidic environment is important for:
- activation of the enzymes responsible for the synthesis of important epidermal lipids,
- formation of the bilayer lipid membrane and
- restoration of the horny layer following mechanical or chemical damage.
An acidic environment is important for synthesis of the epidermal lipids, which consist mainly of ceramides (40%), free fatty acids (25%) and cholesterol (25%). Synthesis of the especially important ceramides is catalysed by an enzyme belonging to the group of acid hydrolases.
- Odland bodies
- Cells of the stratum granulosum
- Bilayer lipid membrane
Compostition and function of the acid mantle
From hydrolipid film to protective acid mantle (i)
Closer examination of the components of the hydrolipid film reveals why this protective film was first named by Schade and Marchionini in 1928 the protective acid mantle (i):
- Sweat contains lactic acid and various amino acids.
- Sebum (i) contains free fatty acids.
- Amino acids and pyrrolidine carboxylic acid are produced by the cornification process.
The physiological pH (i) of healthy skin has an average value lying between 5.4 and 5.9.
In this pH (i) range the skin is populated by a normal skin-typical flora. Pathogenic microorganisms are hindered from spreading. In the armpits, anal folds and the genitals, however, the pH (i) is approximately 6.5 (physiological gaps).
An important mechanism: base neutralizing capacity
A rise in the pH (i) into the alkaline range - for example due to excessive use of soap - disturbs the physiological balance of the skin. If the pH (i) is higher than that of the normal physiological range for an extended period, the function of the bacteriological defence mechanism of the skin is compromised, favouring infections.
To counteract the influence of alkaline substances the protective acid mantle (i) uses what are called buffer substances. They neutralize alkaline substances and ensure the acidic milieu is restored and stabilized. This capability is referred to as the base neutralizing capacity. Alkaline noxae (i) (harmful substances) include substances that act as bases (“alkali-like”, pH (i)>7) in aqueous solution. Examples are soap or sodium carbonate solutions, which can have a pH (i) of up to 11.
The hydrolipid film is made up of several substances:
- Sweat and sebaceous lipids
- Substances derived from the cornification process
- Desquamating but still adhering horny cells
- Water from the deepest layers that has reached the surface
Due to the presence of weakly acidic components, the aqueous portion of the hydrolipid film forms the protective acid mantle (i).
This fulfils three important functions:
- Support of the formation and maturation of the epidermal lipids and hence maintenance of the barrier function
- Indirect protection against invasion by microbial pathogens
- Direct protection against alkaline noxae (i) (base neutralizing capacity)