Invertebrates versus vertebrates innate immunity: In the light of evolution

Invertebrates use a nonadaptive, innate immunity, the expression of germlin e encoded receptors, to identify the allogeneic and xenogeneic attributes. Vertebrates also have the capacity to express ontogenically related adaptiv e immunity which is a somatically selected gene rearrangement process. Seve ral commonly accepted generalizations are utilized to explain the enigmatic lack of the adaptive immunity in invertebrates. All point to the primitive nature of the innate immunity and the primitive organization of the body p lan and the life history patterns of invertebrates. Seven of the most commo n generalizations are reviewed and confuted by virtue of a biased literatur e presentation. Subsequently, three evolutionary puzzles are raised and the accepted paradigm that the vertebrate immunity is pathogenically directed is further challenged. This leads to an alternative idea suggesting that pr eserving the individuality against the threat of invading conspecific cells might have been the original function of the immune system. This ancient s ystem has been co-opted later on to serve as a defence mechanism against pa thogens. The secondary role arose in the form of a multiplicity of newly de veloped phenomena, one of them being the vertebrate adaptive immunity. This proposal is supported by the fact that vertebrates still exhibit two disti nct but common types of naturally occurring transplantation events (natural chimerism) and by a variety of recent studies, providing evidence for the crucial role of the vertebrate's innate immunity in signalling and triggeri ng the acquired effector mechanisms.