One of the biggest threats to survival is infection. To continue existence and to overcome the threat of infection, organisms have developed passive and aggressive defense mechanisms.
Host defense mechanisms have been evolved since the first organisms encountered each other, hundreds of millions of years ago.
Even simple unicellular organisms like bacteria also possess defense mechanisms to get protected against their pathogens (phage virus) that can cause infections.
Complexity of defense mechanism increases with the increased complexity of body, ensuing development of a complex system known as “immune system” in multi cellular organisms.
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Evolution of the immune system is a direct consequence of microbes-exerted selection pressure on multi cellular organisms.
By tracing the evolution of invertebrate immune system, it can be seen that it largely followed bi-directional “predator-prey” relationships. The vertebrate immune system was to some extent inherited from invertebrates, whereas a part of it has advanced considerably in the course of its own evolution.
Although certain vertebrate-specific properties such as immune recognition and immune memory have also been identified in invertebrates in rudimentary forms, it is particularly those qualities like progressive development of humeral and cellular adoptive immunity, Major Histocompatibility Complex (MHC), variable class I and class II genes, precise mechanisms of immune recognition and long-term immune memory, reflect the fundamental evolutionary advancement of the vertebrate immune system.
The mammalian immune system is an incredibly complex and intricate system that can recognize non-self and provide protection from a wide variety of pathogens. In humans, the immune system begins to develop in the embryo.
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By the time a baby is born, the immune system becomes a sophisticated collection of tissues, organs and enzymes. Even the skin is considered as a part of the immune system.