The skin protects against infection mechanically and chemically. Mechanically, the cells of the skin are tightly connected to one another and, for the most part, form an unbroken barrier. Further, the top layer of the skin is composed of keratin, a tough waterproof protein, and dead cells. Chemically, the skin is dry and acidic due to the presence of fatty acids and lactic acid, conditions that are not conducive to the growth of most microorganisms, and contains other inhibitory substances.