The unicellular parasite Giardia duodenalis is one of the most commonly found intestinal pathogens in a wide range of vertebrate hosts, including humans. An infection can present itself with several gastro-intestinal complaints such as diarrhea and abdominal pain. Although in many cases the infection is acute and self-limiting, a significant proportion of infected hosts develop a chronic infection lasting for several months. Given the global importance of Giardia infections in veterinary and public health, significant efforts have been made in recent years to elucidate the host-parasite interaction. The most notable change in our understanding of the immune response elicited by an infection has been the discovery of IL-17A as a crucial regulator in the protective immune response. IL-17A switches on the intestinal production of a range of antimicrobial peptides, complement factors and regulates the production and secretion of parasite-specific antibodies, which ultimately results in parasite clearance. In contrast to our detailed knowledge of the effector mechanisms involved, the processes occuring upstream of IL-17A remain largely unknown. Therefore, the aim of my fellowship is to unravel the upstream signaling cascades following a Giardia infection that lead to the IL-17A response and identify the cells involved in this process.