Immunology has made tremendous progress during more than half century after Burnet's clonal selection theory was published, but there are still more questions than answers. What is the function of the immune system, given that invertebrates have lived without one for millions of years, although they are also susceptible to infections and tumors? On the other hand, the emergence of the immune system in evolution did not deliver higher animals from either infections or cancer. The concept of linked functions is an attempt at answering these and other related questions. The concept assumes that the evolutionary origin of the immune system is related to a primary nonimmune function rather than to self/nonself recognition. However, the mechanisms used to fulfill this function proved to be a suitable basis for immune recognition, which, according to the concept, occurs at the level of receptor-bearing immune cells rather than receptors themselves. Since cross-reactivity is a common phenomenon, it is assumed that specific combinations of antigenic determinants, rather than determinants per se, serve as recognition criteria, antigen processing and MCH-II restriction being necessary steps of the immune recognition of these combinations. The new views on adaptive immunity suggest new approaches to preventing graft rejection and treating chronic infections and malignant tumors.
Keywords: Immune system, evolution, recognition, primary function, secondary function