In recent years the study of pulmonary physiology and pathogeny of pneumonic processes of certain species of mammals are suffering important changes, due to the description of a resident population of cells belonging to the Mononuclear Phagocyte System, the pulmonary intravascular macrophages (PIMs), which form a resident population in the pulmonary capillaries of certain animals, mainly those of the Artiodactyla and Perissodactyla orders. The existence of PIMs was first reported in the early Seventies, but they were not studied in great depth until the late Eighties, when several studies into their features and functional significance were made in different species. It has now been recognized that PIMs are largely responsible for blood clearance in certain animal species, as greater titres of particles, endotoxins and bacteria have been found in lung than in liver or spleen of species in which they are present.

Since PIMs were first reported, most researches carried out has been of a physiological nature, aimed at describing the species in which PIMs are present and the role that these cells play in the hemodynamic control and in the acute inflammatory response at pulmonary level, characterised by recruitment and activation of neutrophils, formation of platelet plugs, and activation of blood clotting. Hemodynamic and inflammatory changes that may be due to the release of various mediators, such as cytokines or arachidonic acid metabolites, as a result of the stimulation of PIMs. There is evidence that, like macrophages from other sources, PIMs release interleukines, tumor necrosis factor, platelet activating factor, leukotrienes and prostaglandin metabolites of arachidonic acid, mediators that can initiate or enhance acute pulmonary inflammation.

In this paper we revise the morphological and functional characteristics of PIMs, with a special interest in their role in pulmonary processes.