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Immune System    
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General Mechanisms

Our bodies, like all organisms, live in a constant balance of trying to get oxygen and food from the environment while trying to protect itself from things that could harm it.  In order to eat and breathe without developing disease, we must defend our bodies against harmful outsiders in food, water and air.  Also to have energy to defend ourselves, we must eat and breathe! 

Why is the immune system needed?

Sometimes, a particle from the outside can enter the body, like a bacteria or virus, and try to harm the cells of your body. Depending on the particular pathogen, it can release toxins that destroy healthy cells (like bacteria) or it can invade particular cells and use the cell’s machinery to replicate itself (like viruses.) Also, some cells in the human body can become damaged or die and become harmful. In these situations, the immune system uses its innate and acquired defenses to rid the body of these unwanted visitors.

The First Line of Defense
Most times, outsiders can be stopped before they can infect cells or replicate.  Your skin alone is a major defense mechanism of the immune system that shields your delicate organs and tissues from infection by posing a physical barrier. Also, hair-like structures called cilia in the mucous membranes of your nose sweep out foreign particles like bacteria, pollen, and dust. (This is why you sneeze!) Tears, mucous, and saliva, along with mucous that lines your lungs, intestines, and urinary and reproductive tracts, all contain enzymes that try to destroy bacteria before they penetrate the body.

The Innate Immune System

If a foreign particle is able to pass past these first defenses, cells will release special chemicals like histamine which heats the blood and causes it to flow faster. Warmer temperatures can kill some bacteria, and the rush of blood will bring white blood cells to the site of infection quicker! 
Learn more about histamine >>

There are many proteins circulating in the blood that make up the complement system that help alert the rest of the immune cells and can also cripple and even kill some invading particles. They can cause infected cells to burst (lysing) and they also release chemicals called cytokines which signal to macrophages that there are cells that need to be eaten. Complement proteins become activated through a cascading effect or a domino effect, where one protein activates the next in a long chain of proteins.

Special white cells called macrophages are swallowing cells that literally engulf and digest any particle that appears foreign. 
Learn more about macrophages >>

The functions of the skin, cilia, bodily fluids, and swallowing cells are all innate defenses of the immune system. 

The innate defenses respond to outsiders as soon as they appear in the body.  Although they are very prompt, they are nonspecific, meaning they aren’t tailored to stop any one particular kind of intruder.  They are like family doctors who give you checkups and prescribe medicine for mild problems.  If they find a serious problem, they will call in the specialists and the surgeons - in this case, the acquired immune system.

The Acquired Immune System


The acquired immune system relies on macrophages, T cells and B cells and other cells working together. These cells concentrate in parts of the body that are more vulnerable to germs, like the very absorbant lining of the intestine.

To survive in our environment, our immune system has developed acquired defenses which depends on special immune cells. These cells are specific, they can distinguish a particular outsider, and are able to learn and remember what particles are harmful. These specialized cells are called T cells and B cells. T cells and B cells remember that foreign particle so that the next time it enters your body, they are prepared to fight it faster.
Learn more about T cells and B cells >>

Together, the innate and acquired defenses of the immune system pose a double threat to pathogens!

Passive Immunity

There is a third kind of immunity: passive immunity. Passive immunity comes from receiving antibodies from another person. This happens between a mother and her child, since they share blood and also during lactation. This protection is immediate and very effective but does not last long.
Antibodies are made by B cells, read about antibodies >>

Major Players and the role of proteins

In these pages we discuss extensively the innate immune system and its swallowing cells (like macrophages and dendritic cells), and the aquired immune system in the form of antibodies and B cells and in the form of Helper and Killer T cells. The less known players in the immune response are all the proteins involved! Proteins act on their own in the body as the complement system, they are involved in the communication between all cells in the immune system, and also play a very important role in identifying each cell or germ.
Learn more about proteins and identifying cells - the antigen >>



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