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==== Front Intensive Care Med Intensive Care Med Intensive Care Medicine 0342-4642 1432-1238 Springer-Verlag Berlin/Heidelberg 10786967 0026S111.134 10.1007/s001340051127 Article Leukocytes: friend or foe Bellingan G grid.13097.3c0000 0001 2322 6764Senior Lecturer in Intensive Care Medicine, Bloomsbury Institute of Intensive Care Medicine, Rayne Institute, University St., London WC1 E 6JJ, UK, , , , GB 2000 26 Suppl 1 S111 S118 Springer-Verlag Berlin Heidelberg 2000This article is made available via the PMC Open Access Subset for unrestricted research re-use and secondary analysis in any form or by any means with acknowledgement of the original source. These permissions are granted for the duration of the World Health Organization (WHO) declaration of COVID-19 as a global pandemic. Leukocytes have a fundamental role in innate and adaptive immunity, wound healing, tumour surveillance and in tissue remodelling. It is their function in the inflammatory response however that is of most interest to us in the intensive care setting. Over the last three decades we have gained significant insights into leukocyte activation, recruitment and mediator secretion and the contribution of these agents to both the onset and resolution of sepsis and inflammation.The body relies on the inflammatory response for protection. Leukocytes occupy a pivotal position in this but to maintain these cells in a state of permanent activation would be unsustainable, with widespread microvascular plugging, uncontrolled free radical release and an excessive metabolic demand. Leukocytes thus circulate in a quiescent state and are rapidly activated by invading pathogens and other stimuli. A direct consequence of this protective strategy is that the inflammatory response may be inadequate, with the risk of overwhelming sepsis, or excessive, leading to rampant systemic inflammation and consequent multiple organ damage.It is now becoming apparent however that in addition to leukocytes other cells have important roles both in defence against invading pathogens and in driving malignant inflammation. This review will focus on two new facets of the innate immune system, the Toll family of proteins as the signal transduction element for endotoxin, and the antimicrobial peptides.

in addition to leukocytes other cells have important roles both in defence against invading pathogens and in driving malignant inflammation. This review will focus on two new facets of the innate immune system, the Toll family of proteins as the signal transduction element for endotoxin, and the antimicrobial peptides. These exemplify potential damaging and protective response elements but importantly neither are restricted to leukocytes. The capacity of cells and tissues other than the leukocytes to participate and even lead in the inflammatory responses will also be explored. Keywords Inflammatory ResponseAntimicrobial PeptideInnate Immune SystemProtective StrategyQuiescent Stateissue-copyright-statement Springer-Verlag Berlin Heidelberg 2000