In an attempt to evaluate the quantitative changes in the status of immunocompetence in severe sepsis over time and its potential influence on clinical outcome, we monitored levels of blood of Immunoglobulins (IgG, IgA and IgM), complement factors (C3 and C4), lymphocyte subpopulations (T, B and natural killer (NK) cells) in 50 consecutive patients with a diagnosis of severe sepsis or septic shock at three moments during their hospitalization in the ICU. The gene discussed is C3; the disease is Sepsis.