Which body region is protected by fatty acids, acidic ph, lactic acid, and a tough cell barrier with its own normal biota?

Skin

The upper skin layer, the epidermis is made of flat cutaneous squamous cells called the keratinocytes that are tightly bound with connective tissue to form a tight barrier. The sebaceous glands in the dermis of the skin produce sebum, a substance that composed of triglycerides (fatty acids) that protect the skin from drying out.
When the sweat glands of the skin secrete amino acids and lactic acid, they mix with sebum to create an acidic micro-environment (ph 5) on the skin that protects against harmful microorganisms.