The immune system: innate and adaptive
immunity
The immune system refers to a collection of cells,
chemicals and processes that function to protect the
skin, respiratory passages, intestinal tract and other
areas from foreign antigens, such as microbes (organisms
such as bacteria, fungi, and parasites), viruses, cancer
cells, and toxins. Beyond, the structural and chemical
barriers which protect us from infection, the immune
system can be simplistically viewed as having two “lines
of defense”: innate immunity and adaptive immunity.
Innate immunity represents the first line of defense to an
intruding pathogen. It is an antigen-independent (non-
specific) defense mechanism that is used by the host
immediately or within hours of encountering an antigen.
The innate immune response has no immunologic
memory and, therefore, it is unable to recognize or
“memorize” the same pathogen should the body be
exposed to it in the future. Adaptive immunity, on the
other hand, is antigen-dependent and antigen-specific
and, therefore, involves a lag time between exposure
to the antigen and maximal response. The hallmark of
adaptive immunity is the capacity for memory which
enables the host to mount a more rapid and efficient
immune response upon subsequent exposure to the
antigen. Innate and adaptive immunity are not mutually
exclusive mechanisms of host defense, but rather are
complementary, with defects in either system resulting in
host vulnerability or inappropriate responses [
1
–
3
].
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