Immune System: Evolutionary Pressure of Infectious Agents

Abstract

Like all other biologic systems, the immune system emerged as a consequence of evolution. A major force that has driven the evolution of the immune system and shaped its current organisation was the needs to fight pathogens. Key elements of adaptive immunity, such as antigen receptors and major histocompatibility complex molecules, show evidence of selective pressures exerted by pathogens. Conversely, immune responses mounted by the host have also imposed strong selective pressures on pathogens. To escape immune attack from the host, pathogens have developed immune evasion tactics such as antigenic variation (as typically seen in influenza viruses) and the production of immune evasion proteins (as typically seen in herpesviruses). Mutation is the key genetic mechanism underlying the co‐evolution of the immune system and pathogens.

Key Concepts:

  • The immune system of vertebrates has evolved to fight pathogens.

  • Pathogens have exerted intense selective pressures on the evolution of the immune system.

  • Key elements of the adaptive immune system, such as antigen receptors and major histocompatibility complex molecules, show evidence of such selective pressures.

  • Immune responses by the host exert strong selective pressures on pathogens.

  • To escape immune attack, pathogens change their antigenicity or produce immune evasion proteins.

  • Mutation is the key genetic mechanism underlying the co‐evolution of the immune system and pathogens.

Keywords: immunity; T cells; antibody; MHC; mutation; pathogen; virus

Figure 1.

The defence systems of animals. IL‐1, interleukin 1; IL‐6, interleukin 6 and TNFα, tumour necrosis factor α.

Figure 2.

Schematic representation of the mammalian immunoglobulin G molecule. Fab, antigen‐binding fragment; F(ab′)2, divalent antigen‐binding fragment; Fc, crystallisable fragment; VH, heavy chain variable domain; CH, heavy chain constant domain; VL, light chain variable domain and CL, light chain constant domain.

Figure 3.

Schematic representation of antigen recognition by T cells. The T‐cell receptor (TCR) recognises a peptide antigen bound to a major histocompatibility complex (MHC) class I (a) or class II (b) molecule expressed by an antigen‐presenting cell (APC). Successful recognition of antigen by a T cell also involves interactions between additional pairs of molecules expressed on T cells and APCs: leucocyte functional antigen (LFA)1–intercellular adhesion molecule (ICAM)1, cluster of differentiation (CD) 2–LFA‐3, CD28–B7‐1/B7‐2 and CD8–MHC I or CD4–MHC II. The peptides of the CD3 complex are involved in signalling T‐cell activation: following recognition of an MHC–peptide complex by the TCR, CD3 transduces signals across the plasma membrane to initiate activation of the T cell. Several of the other molecules also participate in this signal transfer.

Figure 4.

Schematic representation of the pathway by which extracellular protein antigens are taken up by antigen‐processing cells, processed, and then expressed in association with major histocompatibility complex (MHC) class II molecules. HLA, human leucocyte antigen and CLIP, class II‐associated invariant chain peptide.

Figure 5.

Schematic representation of the pathway by which internal protein antigens are processed and expressed in association with major histocompatibility (MHC) class I molecules. TAP, transporter associated with antigen processing.

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Kasahara, Masanori(Jan 2011) Immune System: Evolutionary Pressure of Infectious Agents. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001131.pub2]