Your body is being continuously bombarded by bacteria, viruses and even fungi. Every day, no matter how clean and careful you are, you are exposed to millions of potential pathogens. Fortunately, your body has developed defenses which are able to protect you against most of these attackers. Your immune system has two primary categories. The first are the nonspecific defenses, which protect you against any invader. The second category are defenses which are tailored more specifically toward individual pathogens.
Skin and Mechanical Barriers
The first layer of defense against bacteria and viruses is the skin. The skin is an active, living organ which serves as a very effective barrier to block pathogens from gaining entry in to your body. The outer layers of the skin are made up of cells which are closely packed and filled with a substance known as keratin. This keratin is tough and not able to be broken in to by bacteria or viruses very easily. Of course breaks in the skin, such as cuts, allow pathogens to enter your body.
Other mechanical barriers to infection include various membranes which cover the respiratory, digestive and urinary tracts. These mucosal membranes are structured to allow some things to pass, while blocking the bad organisms. For example, the lining of the respiratory tract contains many tiny cilia, which can be thought of a small hairs. These hairs wave around like small brushes, pushing particles and bacteria out of the lungs and respiratory system. Similarly, your digestive and urinary tracts produce various kinds of saliva, mucus and other secretions which aid in flushing out the invading particles.
Chemicals and Enzymes
If an infectious organism is able to get past the physical barriers, there are other defenses that take over the protection of your body. An entire armament of chemicals are produced by your immune system that are used to kill potential pathogens. Two of the more common of these chemicals are lysozyme and hydrochloric acid.
Lysozyme is found in tears and saliva. It is a chemical produced by cells in your body and is used to attack a wide variety of bacteria. It is a nonspecific immune defense, as it does not require priming by your immune system, nor does a target specific bacteria. It acts by breaking down the cell walls of a many different types of bacteria.
Another defense against invading bacteria and viruses is acid in your stomach. Your stomach contains a relatively high concentration of hydrochloric acid. This makes the pH in your stomach very low and not a particularly good environment for bacteria and viruses to survive in. Many pathogens are destroyed in this acidic environment, however there are many which are resistant, making this an imperfect defense.
Elevated body temperature is another reaction by your body to fight off potential disease. In order to function properly, the cells of your body, as well as foreign bacteria and viruses, must function within a very narrow range of temperatures. Your body contains a complex series of regulatory mechanisms to keep your temperature within this range.
When you are infected by a pathogen, your body responds by raising the temperature a slight bit. This causes a fever, which can be quite uncomfortable not only for you and also for the invading pathogen. The purpose of the fever is to prevent the growth and spread of the invading microbe. Of course, a fever that is too high can be dangerous to your body as well, making temperature regulation a constant balancing act.
inflammation is a very specific medical term which refers to a series of reactions that occur in response to infection or tissue damage. The characteristic signs of inflammation include redness, swelling, warmth at the site of the inflammation, and pain. Inflammation causes your immune system to mobilize, and repair mechanisms within your body to be activated. The inflammatory response is mediated by a wide variety of immune system cells. These responses together serve to help fight an infection of any kind.
Neutrophils and Monocytes
Neutrophils and monocytes are specific types of white blood cells. These cells are mobilized in response to an infection. The act on an invading pathogen by attempting to engulf it - a process known as phagocytosis. Once they gain entry to an infected tissue, monocytes will transform into a type of cell known as a macrophage. Macrophages and neutrophils are first responders to an invading pathogen. They are not picky about what bacteria or viruses they attack, but rather attempt to remove anything that is determined to be foreign by your immune system.
Your immune system is a highly complex and sophisticated series of defenses. Taken together these defenses are extremely effective at preventing you from being infected by the thousands of pathogenic microbes that you are exposed to every day. Each of the individual topics touched on above is only the slightest introduction. The details, mechanisms, and control systems involved in each of these nonspecific immune defenses is quite complicated. However, make no mistake, if you were to not have these nonspecific immune defenses, you would most certainly suffer dire consequences.