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Learning Objective:

Describe how the immune system works.

“First, it is important to recognize that cells, viruses, and other particles have unique molecules and groups of molecules on their surfaces that can be used to identify them. These molecular markers visible to the immune system are called antigens. Human cells have unique cell markers embedded in our plasma membranes that identify each of our cells as self—that is, belonging to us as an individual. Foreign cells or particles have nonself molecules that serve as recognition markers for our immune system. The ability of our immune system to attack abnormal or foreign cells but spare our own normal cells is called selftolerance[1].”

“The body takes a number of measures to prevent infection. The body’s primary defenses against infection include the skin, tears, stomach acid, urine, sweat, mucus, and saliva. By having this range of both physical and chemical defenses, the body is able to defend against a range of pathogens.

Secondary defenses bring about inflammation. The swelling, redness, and warmth of the infected area cause the body to call in macrophages and neutrophils to consume the bacteria. If the pathogen is a virus, interferon (interferon proteins interfere with the ability of viruses to cause diseases) is produced so that other cells in that region of the body can block the virus from attacking any healthy cells.

The body’s third line of defense is the way the body remembers specific pathogens and their structures. If the pathogen enters the body again, the body’s response will be much quicker than the first time the pathogen invaded the body. Antibodies, specific to each pathogen, are ready to respond should this occur.
The memorization and production of antibodies is called active immunity. In passive immunity the antibodies have been obtained from outside the body, either from another animal or person.

A number of cells are involved in combating the invasion of viruses and bacteria. B cells have antigen receptors and antibodies, and they work to fight off bacteria. B cells can form plasma cells and memory cells. The plasma cells produce antibodies that bind to antigens, whereas the memory B cells form new plasma cells if the bacteria enter the body again. T cells are responsible for recognizing nonself cells. On engagement with nonself cells, they produce killer T cells and memory T cells. The killer T cells have the task of binding to cells that have been infected by viruses. The memory T cells are ready to produce more killer T cells if the virus enters the body again. In both cases, bacterial and viral infections, helper T cells are available to recognize the antigens that have been ingested and displayed by macrophages[2].”


1. Thibedeau, G. A., & Patton, K. T. (2006). Anatomy & physiology (6th ed.). St. Louis: Elsevier Health Sciences.

2. Evangelist, T., Orr, T., & Unrein, J. (2009). McGraw-Hill: Nursing school entrance exam book. Department of Defense (DoD).

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David L. Heiserman, Editor

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Revised: June 06, 2015