The Immune System (also known as the Immunological System) is made up of all the mechanisms through which a multicellular organism defends itself from internal invaders such as bacteria, viruses or parasites. The immune system can be divided into two main branches: The innate or non-specific immune system, consisting of physical and chemical barriers such as skin, gastric acid, mucus or tears as well as cells and active mechanisms such as phagocytes, natural killer cells and the complement ...
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