The aims of this project were three-fold: (1) to develop diagnostic assays to identify individuals who were allergic to the Australian paralysis tick, Ixodes holocyclus; (2) to identify, isolate and purify, either wholly or partially, the causal allergen(s) and (3) to use these techniques and knowledge to study the nature of the poorly understood immediate hypersensitivity reactions induced by the tick. In the long term, it was hoped that the research would be of use for diagnostic and clinical as well as research purposes. It was also envisaged that the research would provide the information necessary for the commercial preparation of a pure allergen extract for specific immunotherapy.The broad aims were addressed as follows with the resulting conclusions:(1) Three techniques were developed to detect IgE specific for I. holocyclus whole body extract (WBE). The radioimmunoassay (RIA) was superior to both the radioallergosorbent test (RAST) and enzyme immunoassay (EIA). The RIA clearly discriminated between individuals with and without systemic allergic reactions. It also identified borderline cases - showing local or cutaneous allergic reactions. It appeared to be suitable for diagnostic and research applications.(2) A Western blot technique suitable for the identification of IgE-binding allergens was developed. Results showed that human serum is a potent blocking agent which may have wide application to the immunodetection of antigens.(3) R1A and Western blotting showed that salivary gland extract (SGE) of I. holocyclus contained a more concentrated source of allergens than WBE, demonstrating that tick-bite allergic responses are a result of the introduction of allergens from tick saliva, rather than the tick body, into the feeding lesion.(4) Partial purification of SGE allergens by sodium dodecyl sulphate electrophoresis and isolation by electroelution showed that there were two major proteins of approximately 28 and 35 kilodaltons (kd) which bound significantly more IgE than did other minor proteins.(5) The incorporation of SGE rather than WBE into the RIA and skin prick test improved the diagnostic accuracies of the two assays. Incorporation of the partially purified 28 and 35 kd major allergens to further increase the specificities of the two diagnostic tests appeared promising, but further investigations are necessary to complete this work.(6) Tick-bite reactions, which are well known but poorly defined, were classified into six categories, including four allergic categories analagous to those described for other stinging and biting arthropods: (1) small local reactions; (2) large local reactions; (3) systemic reactions; (4) atypical allergic reactions; (5) tick-borne reactions and (6) tick paralysis. The classification was based on clinical descriptions of a large panel of individuals. SGE-based RIA and skin prick tests were useful for the assignation of individuals into allergic and non-allergic categories. (7) The presence of the 28 and 35 kd major allergens in I. holocyclus as it developed from the egg through to the larval, nymphal and mature male or female adult stage was investigated. Both major allergens occurred in high concentrations in extracts of nymphs and, in particular, the adult female, but in trace amounts only in eggs, larvae and males. It was proposed that the allergens may be feeding-associated enzymes or proteins necessary for the development of the tick. The findings also explained the clinical observation that severe allergic reactions are associated with bites by adult female ticks.(8) In summary, the results of this investigation showed that immediate reactions to I. holocyclus are IgE mediated; that levels of specific IgE can be determined by the RIA developed and put to diagnostic, clinical or research use; that there appear to be two major causal allergens, of which preliminary studies showed that the 28 kd allergen may be of greater importance; that the clinical spectrum of allergic reactions is analagous with other arthropod-induced allergies; that the production of a desensitising preparation would be reasonably simple as there appear to be only two important allergens; and finally, that life-threatening allergic reactions appear to be confined to the adult female tick and possibly the nymph.