Immune System


The immune system comprises an interacting assemblage of cells and soluble molecules, whose primary function is to kill the invading microorganisms that may cause damage to the body. Two interdependent kinds of immune systems are present in most vertebrates, which together trigger one or more different killing mechanisms according to whether the microbes live within or outside the cells of the body. An innate system, mediated by receptors that recognise uniquely microbial structures, responds rapidly to the threat of invading organisms. This underlies an adaptive system, mediated by antigen receptors on lymphocytes, which produces a more sustained and comprehensive response. Only the adaptive system, found exclusively in vertebrates, retains a memory of exposure to each microbe and ensures that the system is mobilised more rapidly on a subsequent infection by the same pathogen.

Key Concepts:

  • A rapidly mobilisable form of defence (the immune system) is required to kill microbes that breach the skin and other passive defences of the body.

  • Physical injury to tissues triggers inflammation, which serves to mobilise the immune system and to repair tissue damage.

  • The innate immune system is mobilised first in response to microbes and is triggered by receptors for microbial components that are not found in the body.

  • The innate system comprises cells such as phagocytes (macrophages and neutrophils) and natural killer cells and soluble antimicrobial components such as complement.

  • The adaptive immune system is mediated by T and B lymphocytes, which are activated through the recognition of microbial components using antigen receptors.

  • The genes for antigen receptors are produced by a unique process of gene rearrangement that results in a vast repertoire of antigen receptors, each with a unique antigen specificity.

  • The repertoire of antigen receptors present in the body ensures that any invading microbe missed by the innate system can be recognised by the adaptive system.

  • The adaptive system uses many of the same killing (effector) mechanisms as the innate system, and is likely to have evolved only in vertebrates.

  • Antibodies are soluble forms of the B lymphocyte antigen receptor that are produced in large quantities during an adaptive immune response; one function of antibodies is to bind to microbes and render them more susceptible to phagocytosis.

  • Because each lymphocyte has a different receptor, the adaptive system is mobilised more gradually than the innate system; however, the response is more sustained, less easily evaded by microbes and unlike the innate system, it retains a memory of exposure to a particular microbe to ensure a more rapid response to the same microbe on subsequent exposure.

Keywords: immunity; antigen; antibody; lymphocyte; infection

Figure 1.

Overview of the acute phase of inflammation. Tissue damage causes mast cell degranulation in the tissues, either directly or via the bradykinin cascade (not shown), thereby releasing histamine and chemotactic factors. Histamine is vasoactive (causes vasodilation and increased vascular permeability) and increases the expression of adhesion molecules, enabling phagocytic neutrophils to adhere and cross into the tissue. Innate immune responses are triggered first. Neutrophils, guided by chemotactic factors, ingest microorganisms by phagocytosis. Increased vascular permeability allows components of the complement cascade to enter, which generates a variety of antimicrobial and proinflammatory substances. Meanwhile, tissue macrophages ingest any microorganisms and in so doing, release inflammatory cytokines. These also cause vasodilation, increased permeability and increased expression of adhesion molecules. Inflammatory cytokines also cause Langerhans cells (not shown) to migrate to draining lymph nodes where they activate T lymphocytes and initiate an adaptive immune response.

Figure 2.

Complement cascades are enzymatic cascades that generate a variety of antimicrobial and proinflammatory substances that help phagocytes to clear away invading microorganisms. The innate or alternative pathway can be activated directly by the cell walls of microbes, or via bound lectins such as the acute‐phase proteins generated during inflammation. The classical pathway is triggered by antibodies bound to the surface of microbes. Despite the nomenclature it is almost certain that the classical pathway has evolved recently than the innate pathways. It provides a good example of an effector mechanism that can be triggered by innate and adaptive forms of recognition. Thereafter, all pathways converge on a common, lytic pathway that culminates in the formation of a MAC.

Figure 3.

Antigen processing and presentation by APCs. Class II MHC molecules are found only on all professional APCs (dendritic cells, macrophages and B lymphocytes). As a general rule, antigens ingested from outside the APC enter this pathway and are processed into peptide fragments by enzymatic degradation in endosomal compartments. These peptides are picked up by class II MHC molecules and presented to CD4 (‘helper’) T lymphocytes. In contrast, class I MHC molecules are found on all nucleated cells in the body (including APCs). These molecules present processed peptides to CD8 CTLs, and antigens presented by this pathway usually originate from within the cell (as is the case in viral infection) and are processed in the cytosol. By this means, all cells have the capacity to be recognised and killed by CTLs should they become infected. There is evidence that dendritic cells can also load class I from antigen derived from outside the cell.



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Davies, D Huw(Sep 2013) Immune System. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000898.pub3]