Food allergy is an IgE mediated reaction of the immune system to food components, most commonly to food proteins. Food allergies develop after primary sensitization through the gastrointestinal tract and the mechanism behind this route of sensitization has not yet been elucidated. It has been postulated that in order to sensitize the organism through an oral route, food allergens must reach the gastrointestinal mucosa in at least a partially intact form in order to interact with the immune system. In addition to the unclear mechanism of sensitization, food allergies also present a challenge in the field of diagnostics, while the only effective therapy method to date is the strict avoidance of allergenic substances. During the last 30 years, since its first appearance on the world market, kiwifruit has become one of the more popular components of human nutrition, and yet, at the same time, it has emerged as one of the ten most frequent elicitors of allergic reactions to food. Actinidin, a cysteine protease from the papain family, is the most abundant protein in the pulp of kiwifruit and is considered the major allergen of this fruit. In this thesis the structural and immunological characteristics of actinidin were examined in order to shed light on the mechanism of oral sensitization and to aid in the improvement of kiwifruit allergy diagnosis. It was shown that actinidin purified from fresh kiwifruit under native conditions represents a mixture of the active and inactive enzyme. Differences in electrophoretic mobility, IgE binding in immunoblot and patient response in skin prick tests were shown for proteolytically active and inhibited actinidin. It was demonstrated that only the active enzyme survives conditions of simulated gastric and intestinal fluid while preserving its immunogenicity and proteolytic activity. The observed differences between the active and inhibited form of actinidin are most likely a consequence of differences in the conformation of the enzyme that arise upon inhibition, a hypothesis supported by results from CD spectroscopy and fluorimetry. Results presented in this thesis contribute to the effort of elucidating the mechanism of oral sensitization to kiwifruit by shedding light on the features of actinidin which enable it to reach the intestinal mucosa in a proteolytically active form. Additionally, since the influence of the activity (conformation) of actinidin on results of in vitro and in vivo diagnostic assays was demonstrated, results from this thesis could contribute to the improvement of kiwifruit allergy diagnosis. In addition, it was shown that the relative ratio of active and inhibited actinidin in kiwifruit protein extract depends on the chosen method for extract preparation. Apart from influencing future kiwifruit allergy diagnostic procedures, these results can be used to optimise the extraction procedure for kiwifruit protein extracts. In this thesis the possibility of using kiwifruit extract as a plant milk clotting agent was analysed. It was shown that the addition of kiwifruit extract prepared at pH 5.0 to cow milk leads to the formation of a casein clot. The obtained clot and whey were shown to have identical protein profiles to the clot and whey obtained through the activity of calf chymosin. The ratio between coagulating and proteolytical activity towards cow milk was only 30 % less favorable when using kiwifruit extract. Alergije na hranu su IgE antitelima posredovane imunološke reakcije na komponente hrane, najčešće proteine. Razvijaju se nakon primarne senzitizacije preko gastrointestinalnog trakta, i za sada nije poznat mehanizam senzitizacije ovim putem. Smatra se da bi alergeni proteini hrane morali da prođu kroz gastrointestinalni trakt barem delimično očuvane strukture kako bi doveli do senzitizacije. Pored nerazjašnjenog mehanizma senzitizacije, alergije na hranu predstavljaju i izazov na polju dijagnostike, dok je jedina do sada potpuno uspešna metoda terapije strogo izbegavanje unosa alergene namirnice. Tokom svega 30-tak godina od svog prvog pojavljivanja na svetskom tržištu pa do danas plod kivija je postao jedna od omiljenijih namirnica u ljudskoj ishrani, ali je istovremeno dospeo i među deset najčešćih uzročnika alergije na hranu. Aktinidin, cistein proteaza iz papainske familije, je najzastupljeniji protein pulpe kivija i smatra se glavnim alergenim molekulom ovog izvora. U oviru ove disertacije ispitivane su strukturne i imunološke osobine aktinidina u cilju rasvetljavanja puta oralne senzitizacije i poboljšanja dijagnostike alergije na kivi. Pokazano je da aktinidin prečišćen iz svežeg ploda kivija pod nativnim uslovima predstavlja smešu aktivnog i neaktivnog enzima. Između proteolitički aktivnog i neaktivnog aktinidina postoji razlika u elektroforetskoj pokretljivosti, sposobnosti vezivanja IgE u imunoblotu i obrascima reagovanja pacijenata u kožnim probama. Pokazano je da samo aktivni enzim preživljava uslove simuliranog želudačnog i crevnog soka i da nakon ovih tretmana pokazuje očuvanu imunogenost i proteolitičku aktivnost. Uočene razlike između aktivnog i inhibiranog oblika aktinidina najverovatnije su posledica razlika u strukturi enzima koje nastaju prilikom inhibicije, a ova pretpostavka je potkrepljena rezultatima CD spektroskopije i fluorimetrije. Rezultati dobijeni u ovoj disertaciji doprinose rasvetljavanju mehanizama oralne senzitizacije na kivi ukazujući na strukturne osobine aktinidina koje mu omogućavaju da u proteolitički aktivnom obliku dospe do intestinalne mukoze. S obzirom da je pokazan uticaj aktivnosti (konformacije) aktinidina na in vivo i in vitro dijagnostičke eseje prikazani rezultati mogu doprineti i poboljšanju dijagnostike alergije na kivi. Pokazano je i da relativni odnos aktivnog i inhibiranog aktinidina u ekstraktu kivija zavisi od načina pripreme ekstrakta. Ovi rezultati, osim u poboljšanju dijagnostike alergije na kivi, mogu biti upotrebljeni i za optimizaciju proteinskih ekstrakata kivija i njihovu upotrebu u industriji mleka. U ovoj disertaciji je ispitana moguća upotreba ekstrakta kivija kao sredstva za koagulaciju mleka u procesu proizvodnje sira. Dobijeni rezultati su pokazali da dodavanjem proteinskog ekstrakta kivija pripremljenog na pH 5.0 u kravlje mleko dolazi do nastanka gruševine i surutke identičnih proteinskih profila kao prilikom dodavanja himozina poreklom iz želuca teleta. Izmeren odnos koagulacione i proteolitičke aktivnost ovog ekstrakta prema mleku je bio svega 30% lošiji od himozina.