"The immune system is a network of cells, tissues,
and organs that work together to defend the body
against attacks by “foreign” invaders.
... The human body provides an ideal environment
for many microbes. It is the immune system’s job to
keep them out or, failing that, to seek out and
destroy them.
When the immune system hits the wrong target,
however, it can unleash a torrent of disorders,
including allergic diseases, arthritis, and a form
of diabetes. If the immune system is crippled, other
kinds of diseases result.
The immune system is amazingly complex. It can
recognize and remember millions of different
enemies, and it can produce secretions (release of
fluids) and cells to match up with and wipe out
nearly all of them."
NIH.gov
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