The first line of defence (or outside defence system) includes physical and chemical barriers that are always ready and prepared to defend the body from infection. These include your skin, tears, mucus, cilia, stomach acid, urine flow, 'friendly' bacteria and white blood cells called neutrophils.
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This is how the cycling between photosynthesis<span> and cellular respiration occurs: </span>in photosynthesis<span>, carbon dioxide and water, </span>in<span> the presence of light energy, are converted to make glucose and oxygen.</span>
In contrast to the stratified squamous epithelium of the skin, the intestinal barrier is composed of a single layer of columnar epithelial cells However, this single layer of intestinal epithelial cells is made of diverse cell types with absorptive, secretory, and immune function