The descent of the antibody-based immune system by gradual evolution

Abstract

The antibody-based immune system (AIS) is one of many means by which organisms protect themselves against pathogens and parasites. The AIS is present in jawed vertebrates (gnathostomes) but absent in all other taxa, including jawless vertebrates (agnathans). We argue that the AIS has been assembled from elements that have primarily evolved to serve other functions and incorporated existing molecular cascades, resulting in the appearance of new organs and new types of cells. Some molecules serving other functions have been appropriated by the AIS, whereas others have been modified to serve new functions, either after the duplication of their encoding genes or through the acquisition of an additional function without gene duplication. A few molecules may have been created de novo. The deployment and integration of the ready-made elements gives the impression of a sudden origin of the AIS. In reality, however, the AIS is an example of an organ system that has evolved gradually through a series of small steps over an extended period.

Fig. 1.

Hypothetical scenario for the emergence of the MHC, TCR, and BCR molecules by gradual evolution, which encompassed modification of preexisting domains, joining together of different domains, and possibly generation of new domain designs.

Fig. 2.

Emergence of the organs, cells, and molecules of the AIS during the evolution of chordates. Black bars indicate the presence of a fully developed trait. Darkly shaded bars indicate the ancestral form of a trait. Lightly shaded bars indicate that an ancestral form of a trait has not been identified.