Antibody-mediated effects on parasite behavior: Evidence of a novel mechanism of immunity against a parasitic protist

The parasitic ciliate Ichthyophthirius multifiliis is well known in commercial aquaculture ns thp etiological agent of 'white spot', a disease that afflicts a wide range of fresh-water Fsh. While Ichthyophthirius is highly pathogenic, animals exposed to controlled infections develop a strong acquired resistance to the parasite. Recent studies suggest host resistance involves a novel mechanism of humoral immunity affecting parasite behavior. Rather than being killed, parasites are forced to exit fish prematurely in response to antibody binning. The target antigens involved in this process a re a class of high ly abundant glycosylphosphatidylinositol-anchored coat proteins referred to as immobilization antigens, or i-antigens. Here, Theodore Clark and Harry Dickerson describe this phenomenon and offer a number of hypotheses that could account for the forced exit.