ant differences compared with healthy volunteers; superscript "b" denotes statistically significant differences compared with patients with flu-like syndrome.](pone.0008393.g002){#pone-0008393-g002}

Concentrations of IL-1β of PBMCs-stimulated supernatants are shown in [Figure 3](#pone-0008393-g003){ref-type="fig"}. No differences were found between healthy volunteers, patients with flu-like syndrome and patients infected by the H1N1 virus.

![Production of interleukin-1beta (IL-1β).\
Peripheral blood mononuclear cells of 10 healthy volunteers, of 18 patients with flu-like syndrome and of 31 patients infected by the H1N1 virus were stimulated with endotoxins (LPS), phytohemagglutin (PHA), and heat-killed isolates of *Candida albicans*, of *Pseudomonas aeruginosa*, of methicillin-resistant *Staphylococcus aureus* producing Panton-Valentine leukocidin (MRSA-PVL) and of penicillin-susceptible *Streptococcus pneumoniae*.](pone.0008393.g003){#pone-0008393-g003}

In order to define if the effect of H1N1 is related with the production of T~H~-1 type cytokines or not, concentrations of IL-6, IL-18, IFNα and IFNγ were also estimated in supernatants of PBMCs stimulated with either PHA or *Streptococcus pneumoniae* ([Figure 4](#pone-0008393-g004){ref-type="fig"}). It was found that stimulation of PBMCs of both patients with flu-like syndrome and H1N1-infection produced greater concentrations of IL-6 compared with healthy volunteers (p: 0.009 for comparisons between flu-like syndrome and healthy volunteers; p: 0.009 for comparisons between H1N1 infection and healthy volunteers). With the exception of single patients, IL-18 and IFNα were below the lower detection limit. Production of IFNγ was greater by PBMCs of H1N1-infected patients after stimulation either with PHA (p: 0.010 compared with healthy volunteers) or with *S.pneumoniae* (p: 0.029 compared with healthy volunteers).

![Cytokine production by peripheral blood mononuclear cells of healthy volunteers, of patients with flu-like syndrome and of patients infected 