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 as the etiological agent of 'white spot', a disease that afflicts a wide range of fresh-water fish. 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 binding. The target antigens involved in this process are a class of highly abundant glycosylphosphatidyl-inositol-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.