title: A perspective on clonal phenotypic (antigenic) variation in protozoan parasites.
abstract: Intra-clonal phenotypic (antigenic) variation is used by many pathogens to evade the consequences of immune-mediated killing by mammalian hosts. In this substantially theoretical article, I emphasise that antigenic variation (sensu stricto) involves no change in genotype; its importance as a mechanism for promoting pathogen transmission and its polyphyletic origin. From a functional perspective, antigenic variation is constrained by the requirement to meet five conditions. These are: capability to express several antigens against which functional immunity predominates; capability to interact with the environment; mutually exclusive expression of variable antigens in each cell within an infection; mutually exclusive expression in the within-host pathogen population and the capability for population growth within a host. Meeting these conditions leads to chronicity of infection and high rates of hierarchical and reversible switching of expression between variable antigens. The organisation of hierarchical expression is discussed in some detail.
authors.affiliation: Division of Infection and Immunity, Institute of Biomedical and Life Sciences, Joseph Black Building, University of Glasgow, Glasgow, G12 8QQ. m.turner@bio.gla.ac.uk
authors.forename: C M R
authors.initials: CM
authors.lastname: Turner
mesh_terms: D000818:Animals; D000940:Antigenic Variation / Q000502:physiology*; D000953:Antigens, Protozoan / Q000276:immunology*; D005075:Biological Evolution; D017125:Genes, Protozoan; D005838:Genotype; D006790:Host-Parasite Interactions / Q000235:genetics / Q000276:immunology; D006801:Humans; D007109:Immunity / Q000235:genetics / Q000276:immunology; D008962:Models, Theoretical; D010641:Phenotype; D011528:Protozoan Infections / Q000276:immunology* / Q000469:parasitology / Q000635:transmission
chemical_list: D000953:Antigens, Protozoan