This protective system we have acquired has evolved from simpler defense mechanisms, but the evolutionary twists and turns that lead a path to the development are not entirely clear. To reveal the path that the human immune system followed in its evolution, researchers have studied the defense responses of various living organisms. They also have examined the genes of the immune system proteins for hints and pieces of evidence to the genetic origins of immunity.
It is, with the help of positions of the animals in the tree of evolution, possible to trace back to the history of the immune system. It is, however, impossible to trace the evolution of immunity from the paleontological record. But since all animals exhibit a few general abilities to recognize self and to repel foreign bodies, it is possible to study the immune capacity of living animals.
As stated earlier, we as vertebrates have the most sophisticated immune system. Only in these organisms, it is possible to trace lymphocytes and immunoglobulins. The world’s most primitive living vertebrates, the jawless fish, do not have lymphoid tissue and their immune response is very weak. Moving forward, the fish with bones lacked lymph nodes but did contain clusters of lymphocytes in the gut that may be useful when required.
Further up the tree of evolution, terrestrial vertebrates – the mammals, the amphibians, etc. inherited a perfect immune system. The thymus, spleen, bone marrow, and lymph nodes were not only present but also made the IgM and IgG antibodies.
The evolution of the complement system (a group of proteins involved in immune responses) may have evolved faster than that of the immunoglobulin system. The fact that the complement system has gotten so well conserved and protected during evolution that has been of great biological value. The complement and immunoglobulin have also interacted throughout the evolution of the immune system in higher vertebrates.