The skins hard keratin layer is effective in stopping entry of micro-organisms. entry can occur through open wounds but blood clotting seals the wound and prevents the number of micro-organisms getting through.
Large numbers of microbes known as skin flora live on the surface of the skin, these are friendly bacteria that prevent the colonisation of other bacteria, it is a mutualistic relationship between human and bacteria. the bacteria that tries to invade the body are usually not well suited to the conditions that the skin provides due to the salty sweat and excreted chemicals like Urea and fatty acids that are on the surface of our skin.
The mucous membranes that line the airways and the gut are easier routes into the body, this is because they are moist and warm and produce more favourable conditions for bacterial growth. But the entry of microbes into the lungs is limited by the actions of mucous and cilia, the mucus which is secreted by goblet cells in the trachea and bronchi traps microbes and other particles, the beating cilia then carries the mucus up to the throat where it is then swallowed.
secretions in the mouth, eyes and nose contains lysozyme, which is an enzyme that breaks down the bacterial cell wall by causing them to burst, this is called Lysis.
Gastric juices secreted by the gastric glands in the stomach walls contains hydrochloric acid which has a pH of less than 2.0 this is enough to kill most bacteria that enters with our food, but it is also the optimum pH for the digestive enzymes.
Bacteria are found in the small and large intestines, these natural flora benefit from living within the gut where conditions are ideal, we also benefit from their presence, they may aid digestive processes and competitively exclude pathogenic bacteria. the bacteria also excrete chemicals such as lactic acid that are useful in the defence against pathogens.