The skin is a fundamental part of the human immune system, in particular of the so-called innate immunity. The latter is so called because it brings together all those physical and biochemical mechanisms, present since birth, that act quickly and almost indiscriminately against any external threat.
The skin belongs to the so-called surface defensive barriers. Just like an armor, in fact, it is able to oppose the penetration of pathogens through defenses:
- mechanical: the most superficial cells of the skin (stratum corneum of the epidermis) are keratinized and tightly packed (they are arranged similarly to the tiles of a roof). Thanks also to the high content of keratin, they constitute a barrier difficult to overcome by microorganisms.
- chemical: the fatty acids of the sebum and the slightly acidic pH of the skin counteract the growth of fungi and bacteria.
- biological: the symbiotic and saprophytic skin flora helps to keep other bacterial species away, including potentially pathogenic ones. Furthermore, a certain amount of lysozyme is present in the sweat, an enzyme capable of destroying the cell membrane of bacteria. Furthermore, at the level of the epidermis (spinous layer), specific immune cells are present, called Langerhans cells . These cells have the particularity of presenting long ramifications (called dendrites), with which they capture the antigens that are then internalized, processed and exposed on the cell surface. In this way, Langherans cells are able to activate other immune cells (T helper lymphocytes) responsible for removing the threat.
