1.2.1　Barrier function

The skin can protect various organs and tissues in the body from damages of external harmful factors（e.g.，physical factors，chemical factors，and microbial factors），and can also prevent the loss of water，electrolytes，and nutrients in the body and maintain the stability of the internal environment.Keratinocytes，keratin desmosomes，keratinocyte lipids，water in the stratum corneum and maintaining an appropriate pH，and enzymes related to keratinocytes shedding are essential for maintaining skin barrier function.

1.Protection against physical damage

The skin has a certain degree of protection against external mechanical damage（e.g.，friction，impact，squeezing，and pulling）.The stratum corneum is dense and flexible，and is the main protective structure.The epidermis can repair itself after being damaged.The dermis contains collagen fibers，elastic fibers，and mesh fibers，which make the skin elastic and stretchable.The subcutaneous tissue has a buffering effect on mechanical external forces.The skin's barrier to electricity is mainly in the stratum corneum and is related to the water content of the stratum corneum.The skin absorbs light and promotes the production of melanin to play a role in light protection.

2.Protection against chemical damage

The stratum corneum is the main structure for protecting the skin from chemical stimuli.In addition，the hydrogen ion concentration on the surface of normal skin can buffer acids and bases.(https://www.daowen.com)

3.Protection against microbial invasion

The stratum corneum cells are densely arranged，and the intact skin can mechanically defend against the invasion of microorganisms.The stratum corneum has less water and the surface pH is weakly acidic，which is not conducive to the growth and reproduction of certain microorganisms.The stratum corneum is physiologically shed so as to clear some microorganisms parasitic on the body surface；the antibacterial peptides produced by keratinocytes have a broad-spectrum antibacterial effect（see extended reading 3）.

4.Prevention of the loss of water，electrolytes and nutrients

The stratum corneum of normal skin is a semi-permeable membrane，which can prevent the loss of nutrients，electrolytes，and water in the body.