Human bodies have a number of strategies to fight infections or prevent them. The whole of our infection fighting apparatus is called the “immune system.” The human body’s immune system doesn’t just include white blood cells, which attempt to catch and destroy germs, but a variety of mechanisms that stop germs from creating infection.
In most cases, humans have certain properties in their bodies that are called innate immunities, allowing bodies to fight infections at virtually all times. For example, the skin, our largest organ, is constantly fighting infection, or warding off infection by acting as a barrier against foreign, non-human cells. Other parts of our bodies, or contents in our bodies are always on guard to fight infections.
The gut and stomach contain mucus that can trap small numbers of foreign bacterial cells, keeping the body from becoming infected. Human bodies use a variety of acids in organs that create hostile environments for foreign cells. We also host helpful bacteria in our bodies that help keep other bacteria entering the body in check.
In addition to these innate immunities, the body begins to work very hard at the beginning of an infection to catch and kill the infection. When foreign bacterial, viral or parasitic cells attempt to take up residence in our bodies, this activates a specific type of white blood cells called neutrophils. You can picture neutrophils as small army bases located in the body that are called into action, and ready to fight infections, when foreign cells appear in the body.
Essentially, when the body encounters infective agents, neutrophils flood the area where the “invasion” is taking place. They can stick to bacteria or fungi, rendering it immovable or useless, or they may release chemicals that kill bacteria. They may also be on catch, destroy and eat missions, in which they fight infections to the death.
Our bodies also learn to recognize infective agents, often making us immune to viruses or bacteria to which we have been previously exposed. The primary actors in what is called acquired immune response are lymphocytes, also a type of white blood cells. Lymphocytes are of two types, called B and T cells and are usually made from our bodies’ bone marrow and thymus gland.
When a recognizable “invader,” something to which the body has already been exposed at a previous time, enters the body, B and T cells travel to the site of the invasion. T cells release proteins that help stimulate the B cells, and can also stimulate the death of our body’s cells to prevent infection from spreading. B cells are “killer” cells that immediately begin to fight infections.
T cells can also release chemicals which cause specialized B cells, called phagocytes, to produce responses in the body that make environments inhospitable for infection. For example, fever is a reaction to the presence of the body’s own phagocytes, and higher temperatures in the body can actually fight infections by “cooking” foreign cells.
These precise actions of the human body to fight infections can be impaired if white blood cell counts are low. This translates to a less effective immune system, which may not be able to fight infections with ease. In some cases, it is necessary to impair the immune system because it misfires. People with autoimmune diseases have an inappropriate immune response to cells that should exist in their body. Some conditions like Lupus and HIV trick immune cells into believing that other cells in the body are “foreign.” This causes cell death and organ dysfunction by “friendly fire.” The body, in other words, attacks itself.
Alternately, when people have transplants, the immune response attempts to destroy transplanted organs because they are foreign to the body. This means an organ recipient must take immunosuppressive drugs to prevent the body from fighting what it perceives as foreign. The downside of immunosuppresants is that they make the body more vulnerable to other foreign invaders, like bacteria and viruses that can cause infections. In other words, shutting down the actions that fight infections in the body, often translates to greater incidence of infections.