This is a very fine, slightly acidic film on the surface of the skin acting as a barrier to bacteria, viruses, and other contaminants that might penetrate the skin.
The acid mantle is formed when the sebum is secreted from the sebaceous glands and is mixed with sweat.
The Ph of the skin is between 4.5 and 6.2 which is slightly acidic while the blood is slightly alkaline (7.4).
Even though the acid mantle helps to fight bacteria from the surface of the skin, some pathogenic bacteria can become adapted to the pH of the skin and may be able to reach internal tissues but will encounter an environment to which they are less well adapted (the alkaline nature of the blood).
This will make them inactive.
This combination of acid exterior and the alkaline interior is one of the body’s non-specific host defenses against bacterial pathogens.