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PHYSICAL AND CHEMICAL BARRIERS
The first-line defence includes barriers to infection, such as skin (peau/pelle/piel) and mucosal surfaces (membrane muqueuse/membrane mucose/membranes mucosas) [E]. Each barrier (skin, the conjuctivae of the eye, or the mucosal membranes of the respiratory, alimentary, or urogenital tracts) has its own protective mechanisms. For example, skin is not only covered by keratin, which forms a tough, water-repellent layer, but is also bathed with oils and moisture from sebaceous and sweat glands. These secretions contain fatty acids inhibitory to bacterial growth. Another example is the low pH in the stomach, that effectively kills many bacteria and eliminates many viruses. Moreover, many mucosal membranes are covered by a protective layer of mucus, which provides a mechanical and chemical barrier formed by a cross-linked, gel-like structure, made up of glycoproteins. It entraps particles and contains anti-microbial enzymes such as lysozyme and peroxidase. It is readily moved by the motion of cilia of the underlying cells.
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