The human body contains formidable defenses against disease. When a patient becomes infected, it is because the body’s defenses were overcome or were unable to recognize the source of the infection. Over the course of a lifetime, the body’s immune system and its defenses will fight off hundreds of thousands of potential infections.
The skin is the largest organ of the body, according to the American Cancer Society, and it is the body’s first line of defense against infection. It acts as a barrier to the entry of disease causing microbes. It fails when it is penetrated by a cut or scrape, something that causes a hole in the skin. Because the skin already has breaks in it due to sweat glands, hair follicles and the entrances and exits of the respiratory tract and the digestive tract, it is an imperfect barrier.
The respiratory tract permits the inhalation of many infectious microbes. Merck notes, however, that it contains a number of defenses, a layered defense, that prevents many potential infections. The nose and mouth maintain both a critical ambient temperature and pH level that some infections germs cannot tolerate. The nose has hairs that act as a barrier, and the mucus lining the nose is sticky to catch and hold germs. Deeper into the respiratory tract, the lungs maintain their own temperature and pH, and the mucus that lines the surface is both a barrier and a trap for infectious microbes.
The digestive tract meets ingested microbes with the acid of the stomach. While some disease causing germs can survive, most cannot and stomach acid kills them. The stomach and the intestines are also coated with mucus which acts as a barrier, and traps microbes. The intestines also contain a variety of friendly microscopic organisms, probiotics, that serve as another defense against infection. They may crowd out the germs, eliminating room to grow, and some will even attack intruding microbes.
Once a disease causing germ, bacteria, virus or fungi, makes it into the body’s interior, it must face the germ fighters in the bloodstream. A piece by the National Institute of Allergy and Infectious Diseases describes how the body produces antibodies against any germ that it has ever been exposed to, by recognizing specific antigens. These antibodies will identify the antigens that they are specific to and latch on to the microorganisms carrying them. They interfere with the ability of the germ to live and reproduce, and they attract the white blood cells, lymphocytes, that entrap and eliminate the germs.
Despite all the defenses that the body has, an infection can still happen. Many of the symptoms of an infection are the body’s last ditch effort to fight the germ. A fever changes the ambient temperature and some microbes cannot tolerate those changes. Vomiting and diarrhea, a runny nose or a cough are all ways the body tries to eliminate the infectious invader.
Like any living organism, the microbes that create an infection live and reproduce within a range of local conditions such as moisture level, temperature, pH and the presence or lack of oxygen. The body erects barriers to prevent the disease causing microbe from entering the body. Once in, the body tries to alter its living conditions. It has a mechanism that permits it to attack known pathogens. For a last resort, it will try to expel the germs.