S: WebMD – https://www.webmd.com/cold-and-flu/immune-system (last access: 16 January 2018); http://www.health.harvard.edu/flu-resource-center/how-to-boost-your-immune-system.htm (last access: 4 September 2014).
N: 1. – immune (adj): mid-15c., “free; exempt,” back-formation from immunity. Latin immunis meant “exempt from public service, free from taxes.” Specific modern medical sense of “exempt from a disease” (typically because of inoculation) is from 1881. Immune system attested by 1917.
– system (n): 1610s, “the whole creation, the universe,” from Late Latin systema “an arrangement, system,” from Greek systema “organized whole, a whole compounded of parts,” from stem of synistanai “to place together, organize, form in order,” from syn- “together” + root of histanai “cause to stand,” from PIE root *sta- “to stand, make or be firm.”
Meaning “set of correlated principles, facts, ideas, etc.” first recorded 1630s. Meaning “animal body as an organized whole, sum of the vital processes in an organism” is recorded from 1680s; hence figurative phrase to get (something) out of one’s system (1900). Computer sense of “group of related programs” is recorded from 1963. All systems go (1962) is from U.S. space program. The system “prevailing social order” is from 1806.
2. The immune system, which is made up of special cells, proteins, tissues, and organs, defends people against germs and microorganisms every day. In most cases, the immune system does a great job of keeping people healthy and preventing infections. But sometimes problems with the immune system can lead to illness and infection.
S: 1. OED – http://www.etymonline.com/index.php?allowed_in_frame=0&search=immune&searchmode=none; https://www.etymonline.com/word/system (last access: 4 September 2014). 2. http://kidshealth.org/parent/general/body_basics/immune.html (last access: 26 September 2015).