The human external defense system consists of the skin and the mucous membranes. The tear fluid secreted by the eyes also contains antibacterial ingredients.

Skin consists of an epidermis and a dermis. Your epidermis is very dense and rapidly regenerating. Only fat-soluble substances are absorbed through it. The top layer of the epidermis is scaly, and it is called the stratum corneum. Melanin-secreting cells (melanocytes), in turn, are located in the lower parts of the epidermis. Melanin causes a person's skin to tan, preventing UV radiation from affecting the deeper layers of the skin.

Skin hairs grow from the dermis and open to the sebaceous (=oil) glands. The sebum secreted by the sebaceous glands makes the skin supple and impermeable to water. If the sebaceous gland becomes blocked, the sebum secreted by it accumulates at the of the sebaceous gland. Bacteria can multiply easily in such a sebum layer, causing an inflammation. This inflamed sebaceous gland becomes a zit or a spot.

The dermis also contains the sweat glands and the skin's sensory organs. Temperature and pressure can be sensed on the skin. Blood vessels run right in the lower parts of the dermis. They bring oxygen, nutrients and heat to the skin.

The skin helps to regulate body temperature. In cold weather, the blood vessels in the skin constrict and heat loss is reduced. In warm weather or during exercise, the blood vessels in the skin dilate and release heat. In addition, heat is removed from the body through the sweat produced by the sweat glands.

The mucous membranes protect the digestive system and the genitals. Like the skin, the mucous membrane is very dense, and its surface can be strongly acidic. In addition, the mucus it secretes collects pathogens that travel with the mucus out of the body. The cells in the stomach wall produce hydrochloric acid, which effectively destroys microbes due to its high acidity.