Categories of parasites, using the broader definition to include fungi and ectoparasites. An ectoparasite (a louse of mice) has recently been found to exert similar immunomodulatory effects to those associated with gastrointestinal helminth infection in a wild wood mouse population [2]. Shown are, clockwise from top left, Leishmania mexicana protozoal promastigotes (L. Prieto-Lafuente); Candida albicans (from ); Ixodes hexagonus (from Wikipedia [15]); and Heligmosomoides polygyrus adult nematode worms (C Finney).

Schematic diagram of the polymorphic elements in immune responsiveness and where pathogen immunomodulation has driven evolution. In black bold type are the immune system families that have diversified primarily at the level of receptor-ligand specificities; those loci in red italic type are loci encoding cytokines, transcriptional regulators and cell surface molecules that are generally polymorphic in promoter, intronic and 3' UTR sequences suggesting a regulatory or quantitative effect of polymorphism. Loci above the bifurcation generally determine T cell activation, and those below down-regulation, although the distinction is blurred: for example, IL-2 promotes both effector T cell proliferation and Treg survival. CLR, C-type lectin receptors, which recognize conserved glycans of pathogens; NLR, NOD-like receptors, intracellular receptors that recognize pathogen products; TLR, Toll-like receptors, which recognize conserved molecular ligands from pathogens; MHC, major histocompatibility molecules, which bind peptide fragments of pathogen proteins and display them for recognition by T cells; TCR, T cell receptor, the highly variable receptor through which T cells recognize their targets.