Leukocytes, the Janus cells in inflammatory disease

Leukocytes, also called white blood cells, can be categorized into three main groups, granulocytes, monocytes, and lymphocytes, which can be further classified into various subgroups. Lymphocytes are known to intervene in immune responses such as secreting cytokines, killing cells, or the production of antibodies. Monocytes/macrophages participate in chronic inflammation by synthesizing numerous mediators and eliminating various pathogens.

The main type of granulocytes is the neutrophil, also called the polymorphmononuclear (PMN) leukocyte; these are usually not found in normal "healthy" tissue and are referred to as 'the first line of defense' against invading pathogens. However, besides the beneficial microbicidal activity of neutrophils, this cell type is also involved in the pathophysiology of organ damage in ischemia/reperfusion, trauma, sepsis, or organ transplantation. The exact role or function of leukocytes during inflammatory processes is far from being elucidated and can only be estimated from the enormous amount of literature on these cell types. The present review will focus mainly on PMN leukocytes and their ambiguous role in normal and inflamed tissue.