What features of the skin and mucous membranes determine their barrier and protective functions?

The outer layer of the skin – the epidermis – is represented by a stratified squamous epithelium. Living cells of the germinal layer of the epidermis intensively divide and advance to the surface layers, where they keratinize, die, and desquamate. Together with desquamous keratinized cells from the surface of the epidermis are removed microorganisms and products of their vital activity.
The secretion of sweat glands contains low molecular weight organic acids. The acidic environment of sweat fluid (pH ≈ 5.5) creates unfavorable conditions for the spread of bacteria and fungi on the skin. The secretion of the sebaceous glands also contains organic substances that adversely affect microorganisms. In addition, the fat secretion protects the skin from drying out and cracking.
Representatives of normal skin microflora constantly live on the surface of the skin, which can act as antagonists of pathogenic microorganisms, preventing their introduction and reproduction. Thus, due to the combined action of a number of protective mechanisms, healthy skin is a serious barrier to the penetration of pathogenic microorganisms.
The barrier and protective functions of the mucous membranes are caused by secretions containing mucus and a number of biologically active substances, as well as the high ability of cells to regenerate. The composition of the secrets of most mucous membranes includes the bactericidal protein lysozyme. Hydrochloric acid and digestive enzymes secreted by the cells of the gastric mucosa, as well as bile components entering the lumen of the small intestine also have bactericidal properties.