Macrophages inflammatory exudate may be in different developmental stages, including resident macrophages already residing in the tissues or body cavity before the onset of the inflammation, monocytes recruited from the circulation that differentiate into exudate macrophages, and monocyte precursors (i.e. dividing mononuclear phagocytes derived from the bone marrow).

Since the cells in an inflammatory exudate can be quite heterogeneous, they should be characterized cautiously. To indicate the degree of heterogeneity of the population of cells in the exudate, it is essential to report not only qualitative but also quantitative data for each characteristic.

The terminology applied to the cells in an inflammatory exudate is generally unclear. For example, terms such as 'stimulated', 'activated', 'elicited' and 'induced' are used interchangeably. To deal with this problem, the following définitions can be used:

1. Resident macrophages. Macrophages present at any given site in the absence of exogenous or endogenous inflammatory stimulus. These cells may occur in an inflammatory exudate as a sub-population because they were already present before the stimulus was applied. They are also called 'normal macrophages'.

2. Exudate macrophages. Macrophages occurring in an inflammatory exudate.

3. Exudate-resident macrophages. These cells are a transitional form between resident and exudate macrophages.

4. Activated macrophages. Macrophages with increased functional activity induced by a given stimulus in vivo or in vitro. Activation thus implies a new functional activity of a cell or an increase in one or more of these activities. Before activation, these cells may have been resident or exudate macrophages. Explicit mention should be made of how the macrophages were stimulated and how activation was measured.

5. Elicited macrophages. Macrophages attracted to a given site by a given substance. This term only indicates the accumulation of (exudate) macrophages at a particular site, and does not refer to the developmental stage of the cells or their functional state. An elicited population of cells is usually heterogeneous in these respects. Since 'elicited' and 'evoked' mean the same thing, use of the latter term is acceptable.

The term stimulated macrophage is imprecise, because stimulation means the application of a stimulus that may result in the elicitation and/or acti vation of cells. This adjective should not be used. The term induced macrophage is also inexact and should not be used, since induced can imply either elicited or activated.

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