Critical disease caused by the new 2009 pandemic influenza virus (nvH1N1) is a challenge for physicians and scientist. As evidenced in SARS and H5N1, the development of an effective immune response plays a key role to overcome viral diseases. We studied host`s gene expression signatures, cytokine and antibody responses along the first week of hospitalization in 19 critically ill patients with primary nvH1N1 pneumonia and two degrees of respiratory involvement. Presence of comorbidities and absence of immunosuppresory conditions were the common antecedents in both groups. The most severe patients (n=12) showed persistant respiratory viral secretion, increased levels of pro-inflammatory cytokines and chemokines in serum, and elevated systemic levels of two immunosuppresory cytokines (IL-10 and IL-1ra). Both groups were able to produce specific antibodies against the virus. The average day for antibody production was day 9 in the course of the disease, defining an early period of innate immunity and a late period of adaptive immunity. The most severe group evidenced a poor expression of a set of MHC class II and T cell receptor (TCR) related genes participating in antigen presentation and cell mediated immune responses in the late phase. 7 patients of this group finally died. This findings evidence that, as observed in sepsis, severe H1N1 disease course with immunoparalysis, which could explain the poor control of the virus along with the increased incidence of bacterial superinfection observed in these patients.