Immunology: Comparative Immunology of Mammals

Abstract

The mammalian immune system comprises a complex, coordinated and finely controlled series of interactions involving cells and molecules which has evolved to protect the host against disease. Mammals consist of a highly diverse group of animals in which the immune system has been subjected to a variety of selective pressures. This is reflected in differences in the organisation and function of their immune systems, and is seen especially in those gene families characterised by complexity and polymorphism, such as those encoding immunoglobulins (Ig), T‐cell receptors (TCR), major histocompatibility complex (MHC) molecules and natural killer (NK) cell receptors. The evolution of these components in a range of mammalian groups and species will be the main focus of this article.

Key Concepts:

  • The mammalian immune system has evolved to protect the host against disease.

  • The system is complex, involving specialised tissues, cells, soluble mediators and membrane‐bound molecules.

  • The mammalian adaptive immune response is characterised by extreme specificity in terms of antigen recognition.

  • This specificity is largely determined by genes encoding the major histocompatibility complex (MHC) molecules, immunoglobulins (Igs) and T‐cell receptors (TCRs).

  • Selection pressure by pathogens is believed to drive the generation of polymorphism in functional MHC genes.

  • MHC molecules present antigen to TCR and some additionally interact with NK cell receptors.

  • NK cells act as both effector and regulatory cells of the immune response.

  • Different mammalian groups and species have evolved a wide range of strategies to cope with rapidly changing pathogens, all aimed at protecting the host from disease.

Keywords: immunity; mammals; immunoglobulin; lymphocytes; evolution; NK cell receptors; MHC; TCR

Figure 1.

Components of the mammalian adaptive immune system. (a) Three components have evolved to provide specificity and diversity in the mammalian adaptive immune response. MHC class I and class II molecules consist of two molecules, but differ in structure. The MHC class I α chain consists of three domains and is associated with β2‐microglobulin, constant to all MHC class I molecules. MHC class II molecules consist of two chains each with two domains. Areas of high variability in the amino acid sequence of the chains relate to the peptide‐binding grooves, as indicated in red. The T‐cell receptor (TCR) consists of α and β chains that have a constant and variable (V) region. Note that some T cells express the γδ chains as an alternative. Immunoglobulin molecules consist of two heavy (H) and two light (L) chains, each with constant and variable regions. Hypervariable regions of the TCR and immunoglobulin that recognise antigen and confer specificity are shown in red. (b) Cellular expression of these components. Antigen‐presenting cells synthesise MHC class I and class II molecules, but not the TCR or immunoglobulin. T cells synthesise the TCR and MHC class I molecules, but not class II (unless activated, and only then in certain species as shown in Table ). B cells synthesise immunoglobulin, MHC class I and class II molecules. Immunoglobulin is expressed on the cell surface and is also secreted as an effector molecule. Polymorphism and specificity combine to generate effective adaptive immunity. There is co‐dominant expression of MHC alleles. Combined with (usually) multiple MHC loci, each cell expresses an array of MHC molecules on its surface capable of binding a wide variety of peptides. It should be noted that the diversity in the TCR and cell‐surface antibody molecules is expressed in a different manner. T and B cells are clonal and express molecules with a single specificity, so diversity arises from the number of different clones. This has evolved to ensure a high degree of specificity in the effector immune response.

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Web Links

http://www.ebi.ac.uk/ipd/mhc

http://www.elsevier.com/wps/find/journaldescription.cws.home/496/description#description

http://www.springer.com/biomed/immunology/journal/251

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How to Cite close
Ellis, Shirley A(Aug 2012) Immunology: Comparative Immunology of Mammals. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001284.pub3]