Evolution of Adaptive Immunity


Historically, two components of immunity were identified, innate and adaptive. Innate receptors recognise molecular patterns common for groups of pathogens or for common pathological changes in host cells. All animals are protected by innate immunity. In addition, in vertebrates, adaptive immunity evolved that is based on the limitless somatic diversification of lymphoid receptors and selective expansion of those receptors that match the antigen/pathogen. Moreover, somatic hypermutation of antibodies combined with antigen selection allows fine‐tuning of an antibody sequence to match the antigen.

The repertoire of adaptive immune receptors in each individual differs in dependence on its life history and provides a basis for a strong memory response. Adaptive immune systems in jaw and jawless vertebrates are built on completely different receptors; immunoglobulin superfamily in jaw vertebrates and molecules with leucine‐rich repeats in jawless vertebrates. However, they are strikingly similar functionally pointing to similar evolutionary forces shaping these systems.

Key Concepts:

  • Both adaptive and innate strategies are used in organisms from bacteria to humans.

  • Innate mechanisms provide basic protection based on evolutionary conserved features of pathogens whereas adaptive mechanisms provide immunity that is shaped by the life history of every individual.

  • Adaptive immunity of jaw and jawless vertebrates is based on different receptors but functionally similar pointing to the same evolutionary driving forces.

  • Combination of adaptive and innate mechanisms is necessary to provide the most comprehensive immune protection.

  • Genome duplications played a crucial role in the origin of the adaptive immune system.

  • Adaptive immune system coevolve with innate immune mechanisms.

Keywords: T cell; B cell; antibody; rags; AID; VLR; rearrangement; diversification; thymus

Figure 1.

The organisation of a hypothetical mammalian immunoglobulin heavy chain locus is shown. At the first step, one of the D segments rearranges to one of J segments. Next, one of V segments rearranges to DJ. All intervening sequences are excised. The resulting recombined gene is expressed as VDJCμCδ message and it is alternatively spliced producing μ or δ chain. After antigen exposure, any of the downstream constant regions can replace Cmu through class switch recombination.

Figure 2.

VLR receptors are encoded as an empty cassette containing only flanking gene segments. Alongside it, hundreds of LRR modules are encoded in different orientations. They are brought into the cassette in stepwise fashion by gene conversion, which presumably is mediated by the enzymes of cytosine deaminase family. CP, connecting peptide; LRR, leucine rich repeat modules; SP, signal peptide.

Figure 3.

Two whole genome duplication events are thought to play a vital role in the evolution of vertebrate adaptive immunity. The most accepted view is that the first duplication took place before splitting of vertebrate lineages whereas the second took place in jaw lineage. However it may be that both duplications happened before the split. Genes similar to components of TCR and BCR are present in lamprey whereas genes similar to VLR are found in zebrafish suggesting that elements of both systems were present in the vertebrate ancestors.



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Further Reading

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Paul WE (2008) Fundamental Immunology. Philadelphia: Lippincott Williams & Wilkins.

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Danilova, Nadia(Apr 2013) Evolution of Adaptive Immunity. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0024598]