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The human immune system is our protector — its job is to defend the body against diseases and other damaging foreign bodies.
The system works by first identifying alien bacteria, viruses, fungi and parasites lurking in the body, and then sends in its troops — white blood cells — to destroy the invaders and the tissues they infect.
Here are 11 surprising facts about the immune system.
Some people have little to no immune systemSlide 2 of 23
Some people have little to no immune system
The 1976 film, "The Boy in the Plastic Bubble," depicts a person with a deficient immune system, who must live out his life in a completely sterile environment because his body is unable to fight infections. Though the story is fictional, the immune system disease — severe combined immunodeficiency (SCID), or "bubble boy disease" — is very real, occurring in about 1 in every 100,000 births.
Bone marrow transplants from a matching sibling donor used to be the only treatment available for patients with SCID, but gene therapy has also recently proved promising.Slide 3 of 23
People long believed fluid imbalances caused diseasesSlide 4 of 23
People long believed fluid imbalances caused diseases
The germ theory of disease, which correctly states that microorganisms cause some diseases, gained prominence in the 19th century. Before germ theory, humorism (or humoralism) dominated Western medical thinking for some 2,000 years.
The discredited theory proposed that the human body is composed of four liquid substances, or humors: blood, yellow bile, black bile and phlegm. A surplus or deficiency in one or more of these humors causes diseases and disabilities. Disease treatments — such as bloodletting — focused on attempting to restore fluid balance.Slide 5 of 23
The earliest known reference to immunity goes back over two millenniaSlide 6 of 23
The earliest known reference to immunity goes back over two millennia
The first vaccine was developed in the late 18th century, but people recognized the importance of immunity long before that.
During the plague of Athens in 430 B.C., the Greeks realized that people who had previously survived smallpox didn't contract the disease a second time. In fact, these survivors were often called upon to attend to those afflicted with smallpox, according to a 1998 article in the International Journal of Infectious Diseases.
In the 10th century, Chinese healers began blowing dried smallpox scabs into the noses of healthy patients, who then contracted a mild form of the disease — and the patients who recovered became immune to smallpox. This practice, which was called variolation or inoculation, spread to Europe and the New England in the 1700s.Slide 7 of 23
Disease symptoms are sometimes the result of your immune system doing its jobSlide 8 of 23