Evolutionary immunology studies the evolution of immunity and the immune system over ages among animals and plants. The main areas of study include:

• the study of relationships between nonspecific (innate) and specific (acquired, or adaptive) immunity
• the study of how graft rejection occurs
• the study of occurrence and historical development of T- and B-systems of immunity

Evolutionary immunology also looks at different cell types of the immune system and the historical development of the ability to recognize foreign antigenic material.

How did the immune system originate?

It is known that life on Earth in the beginning was in the form of single-cell organisms more than 3.5 billion years ago. With development of multicellular organisms the continuous interactions between the diverse species was both in terms of cooperation and competition. The struggle to survive led to the evolutionary refinement of intricate immune systems. This included the development of both the adaptive and innate immune system.

How do we know about the evolution of the immune system?

Evolutionary immunology clues have been obtained from studies of humans and mouse models. The core concepts of innate immunity for example have been noted across species of animals.

The observations of phagocytosis by mesenchymal cells in starfish and sea urchins led to the original conceptualization of self versus non-self recognition.

Birds provided clues to cellular and humoral immune function to the separate T and B lineages of lymphocytes and also led to the discovery of genes and their specific receptors involved in adaptive immunity. It was experiments on birds that gave clues to gene conversion and diversification of antibodies.

Now the concept of Toll receptors as crucial mediators of innate immunity has been developed from studies in insects. These animal studies provide clues to evolutionary immunology as well.

Innate and adaptive immunity

In the history of development of immunity it has been seen that innate immunity preceded adaptive immunity in the evolution of immune recognition and it is the common link between immune recognition in invertebrates and vertebrates.

It is in vertebrates that different clones of lymphocytes against specific antigens are seen. The vertebrates also have antigen-specific receptors of two types. Those in jawed vertebrates, sharks to humans, are based on the rearrangement of segmental elements encoding the immunoglobulin domains in T and B cell receptors mediated by recombinase-activating proteins RAG-1 and RAG-2. Jawless vertebrates such as lamprey and hagfish have a system that works in a RAG-independent way.

Among invertebrates immune-type receptors may undergo somatic differentiation through RNA processing mechanisms. This helps them achieve adaptive protective immunity and not true immunological memory. Polymorphic diversification for self-non-self recognition is seen even in marine invertebrates