Long pentraxin 3 (PTX3) is a conserved pattern-recognition secreted protein and a host-defence-related component of the humoral innate immune system. The aim of the present study was to characterize swine PTX3 (SwPTX3) protein expression in influenza virus infected pigs. First, we performed in silico studies to evaluate the cross-reactivity of PTX3 human antibodies against SwPTX3. Secondly, we used in vitro analysis to detect SwPTX3 presence in swine bone marrow dendritic cells (SwBMDC) upon stimulation with different agents by Western blot and immunofluorescence. Finally, the levels of SwPTX3 were assessed in experimental infection of pigs with different strains of influenza virus. This is a novel study where the expression of SwPTX3 was evaluated in the context of a pathogen infection. The initial characterization of SwPTX3 in influenza virus infected pigs contributes to understand the role of PTX proteins in the immune response. ; This work was partly funded by the projects AGL2009-12945-C02-01 and AGL2010-22200-C02-01 by the Spanish Government and FP-7-2008-1, 228394, NADIR project funded by the EU. The contribution of Regione Lombardia (project Metadistretti–SEPSIS), the European Research Council (project HIIS) and the Fondazione Humanitas per la Ricerca is gratefully acknowledged.