Since their creation, living organisms, particularly animals, have fed on other living organisms as the source of nutrition. Therefore, all living organisms are required to constantly judge other organisms they encounter and determine whether they are potential food, a predator or potential danger, or an organism with which it can coexist peacefully. This judgment system has become more and more complex and sophisticated as the size and types of living organisms became diverse through biological evolution and some organisms started to live within the body of other organisms. The defense reactions to such foreign organisms in the body are called biophylaxis. Among higher multicellular animals, this is known as the immune system that protects the body from parasitic small organisms. As unicellular microorganisms have short life cycles and evolve very quickly, they rapidly develop methods to overcome the defense systems of higher multicellular animals. To deal with this, the immune system has itself evolved into perhaps one of the most complex and unique physiological systems present in multicellular higher animals. Mechanisms such as cell response, cell differentiation, cell locomotion, and cell growth are abundant in the immune system. Furthermore, novel mechanisms found only in immune cells have developed during the course of evolution.

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