Physical barriers are the very first level of immunological protection. The first and most obvious of these is, of course, our skin. There are very few pathogens that can penetrate our intact skin. They have to rely on cuts and wounds to penetrate the body hence why cuts heal so rapidly. The structural integrity of the skin needs to remain intact. As well as being a physical barrier, our skin has other means of warding off pathogens. Sebum that is excreted by the skin contains fatty acids that inhibit microbe growth and the surface of the skin is acidic making it an unfavourable surface for many pathogens. The skin’s surface is also home to a vast bacterial colony that lives symbiotically with us. These bacteria are protective of their homes and initiate aggressive responses to other bacteria and pathogens that may compete with them.
Our skin only covers our body, but there is much more surface area that needs the protection of a physical barrier. Inside the body, many organs and tissues are protected by mucous membranes that have a secretory function providing a further stage of protection.
Mucosal barriers are also important and act by trapping microbes and foreign particles. In the respiratory tract, the mucociliary escalator mechanism moves mucus to the throat where it can be swallowed and the nose has hair that filters microorganisms.
Other secretions within the body flush out microbes such as tears in the eyes, saliva in the mouth and perspiration on the skin surface. All of these contain the enzyme, lysozyme, that breaks down bacteria. Vaginal secretions are slightly acidic to discourage bacterial growth and gastric acid destroys pathogens in ingested substances.
Reflex action such as coughing and vomiting are also part of the immune system.