The pathophysiological effects of severe sepsis, septic shock and related syndromes result from tissues damaged by the uncontrolled production of the mediators of inflammation. Early deaths are related primarily to the acute effects of the systemic inflammatory response. Later deaths are related more closely to the consequences of multiple organ dysfunction. Monoclonal antibodies and other immunotherapies have been developed against bacterial products, cytokines and other mediators involved in this systemic inflammatory response. Immunotherapies may improve outcome in the critically ill with sepsis if used early and as part of the therapeutic regimen of antimicrobial agents and intensive care support.