immune response to serious injury is one of inflammation, which is usually termed the systemic inflammatory response syndrome (SIRS). In patients with major traumatic or thermal injury, SIRS is evident clinically shortly after initial resuscitation. In a significant minority (25–35%) of patients, SIRS persists without respite and may lead directly to the multiple organ dysfunction syndrome (MODS), particularly if infection supervenes [1,2]. In the majority of patients SIRS remits after several days without progression to early MODS. However, the patient may be left with an increased susceptibility to nosocomial infection associated with production of anti-inflammatory mediators by the immune system, a process referred to as the compensatory anti-inflammatory response syndrome (CARS) [3]. Invasive infection occurring during this period may again induce systemic inflammation, which in turn may lead to the multiple organ dysfunction syndrome (late MODS). Organ dysfunction both early and late is associated with substantial mortality [1,2]. Since cells of the innate immune system are the chief source of inflammatory mediators and cellular and humoral components of innate immunity are major effectors in the early defense against invading microorganisms, it is not surprising that the majority of research into the immune consequences of injury has focused on the innate immune system. The adaptive immune system, by contrast, ordinarily responds only when activated by antigen-presenting cells of the innate immune system. Whereas the T-cell receptors and antibodies of the adaptive immune system demonstrate amazing diversity and exquisite precision for individual antigenic epitopes, several days of clonal expansion following initial activation are ordinarily required for a significant response to become evident. Thus, at first glance, the adaptive immune system appears to be ill suited to play a role in host defense early after injury, although later participation might be anticipated. Yet, the macrophages and dendritic cells of the innate immune system and the Tand B-lymphocytes of the adaptive immune system are in intimate contact with one another in the secondary lymphoid organs and mucosa-associated lymphoid tissue. Thus, interactions between the two immune systems are continuous and likely to be of biological significance after injury as well as in the uninjured state. I will attempt to summarize the available evidence that the T-cells and B-cells of the adaptive immune system and their products participate along with the innate immune system in both the SIRS and CARS responses following major injury. The pivotal cell in the adaptive immune sysSURGICAL INFECTIONS Volume 3, Number 4, 2002 © Mary Ann Liebert, Inc.